Category Archives: Announcements

Announcements affecting the Xen Project community

Xen Project 4.9.2 are available!

I am pleased to announce the release of 4.9.2. Xen Project Maintenance releases are released in line with our Maintenance Release Policy. We recommend that all users of the 4.9 stable series update to the latest point release.

These releases are available from their git repositories

xenbits.xen.org/gitweb/?p=xen.git;a=shortlog;h=refs/heads/stable-4.9 (tag RELEASE-4.9.2)

or from the XenProject download page

www.xenproject.org/downloads/xen-archives/xen-project-49-series/xen-492.html

These releases contain many bug fixes and improvements. For a complete list of changes, please check the lists of changes on the download pages.

Join us at Root Linux Conference Happening in Kyiv, Ukraine This April!

Root Linux Conference is coming to Kyiv, Ukraine on April 14th. The conference is the biggest Linux and embedded conference in Eastern Europe with presenters exploring topics like: Linux in mobile devices, wearables, medical equipment, vehicles, and more. Want to learn about the next generation of embedded solutions? This is the conference for you.

Juergen Gross, Linux Kernel developer at SUSE, and Paul Durrant, Senior Principal Software Engineer at Citrix Systems, are keynoting the conference. Juergen will cover Xen paravirtualized (PV) devices and Paul will cover Intel GVT-g integration into XenServer.

Early bird priced tickets for the event are still available here.

See you there!

Call for Proposals Open for the Xen Project Developer and Design Summit Happening in June!

Registration and the call for proposals are open for the Xen Project Developer and Design Summit 2018, which will be held in Nanjing Jiangning, China from June 20 – 22, 2018. The Xen Project Developer and Design Summit combines the formats of Xen Project Developer Summits with Xen Project Hackathons, and brings together the Xen Project’s community of developers and power users.

Submit a Talk

Do you have an interesting use case around Xen Project technology or best practices around the community? There’s a wide variety of topics we are looking for, including cloud, server virtualization, unikernels, automotive, security, embedded environments, network function virtualization (NFV), and more. You can find all the suggested topics for presentations and panels here (make sure you select the Topics tab).

Several formats are being accepted for speaking proposals, including:

  • Presentations and panels
  • Interactive design and problem solving sessions. These sessions can be submitted as part of the CFP, but we will reserve a number of design sessions to be allocated during the event.
    • Proposers of design sessions are expected to host and moderate design sessions following the format we have used at Xen Project Hackathons. If you have not participated in these in the past, check out past event reports from 2016, 2015 and 2013.

Never talked at a conference before? Don’t worry! We encourage new speakers to submit for our events!

Here are some dates to remember for submissions and in general:

  • CFP Close: April 13, 2018
  • CFP Notifications: April 30 – May 2, 2018
  • Schedule Announced: May 3, 2018
  • Event: June 20 – 22, 2018

Registration

Come join us for this event, and if you register by May 2, you’ll get an early bird discount of $125/ 800 Yuan Travel stipends are available for students or individuals that are not associated with a company. If you have any questions, please send a note to community.manager@xenproject.org.

Curious about last year’s event? Check out a few of our presentations last year here!

Xen Project Community Spotlight: DornerWorks

The Xen Project is comprised of a diverse set of member companies and contributors that are committed to the growth and success of the Xen Project Hypervisor. The Xen Project Hypervisor is a staple technology for server and cloud vendors, and is gaining traction in the embedded, security and automotive space. This blog series highlights the companies contributing to the changes and growth being made to the Xen Project, and how the Xen Project technology bolsters their business.

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Name: Robert VanVossen
Title: Embedded Engineer
Company: DornerWorks

When did you join the Xen Project and why/how is your organizations involved?DornerWorks has been involved with Xen Project since 2011 when we submitted the ARINC653 scheduler to the mainline. Through a Small Business Innovation Research (SBIR) contract from the US NAVY, we worked to develop some ARINC653 extensions to the Xen Project Hypervisor. This led to DornerWorks gaining expertise in the Xen Project Hypervisor and we combined this expertise with our knowledge of the embedded world to help our customers design the embedded virtualization solutions they need. This formed the basis for our Virtuosity product (a hypervisor distribution based on the Xen Project) and design services around embedded hypervisors.

DornerWorks still tries to propagate changes and bug fixes to the Xen Project Mainline whenever we can as we take great pride in being a part of this community. We want to help improve both the technology and the community through our work.

How does your involvement benefit your company?
A hypervisor is a complex piece of technology and DornerWorks is a small company.  By colloborating with the larger Xen community, DornerWorks is able to provide a competitive embedded virtualization solution without needing to become an expert at all the technology areas encapsulated in Xen Project technologies.

As a primarily services based company, the Xen Project community also provides us with an indirect marketing platform. The opportunity to publicly showcase our contributions and present on technical topics at Xen Project conferences allows us to share our expertise with the community while getting our name in front of potential customers.

The Xen Project community has also been instrumental in encouraging Xen’s use in embedded platforms, which while different from Xen’s original cloud based goals, is an area poised for growth in virtualization implementations.

How does the Xen Project’s technology help your business?
Xen Project technologies provide a basis for services that we provide to our customers. Through the DornerWorks Virtuosity distribution (http://dornerworks.com/xen/virtuosity), we give customers the means to get started quickly prototyping with Xen on embedded systems. From there, we provide services to refine their solution or develop new components around Xen that they may need, such as new guest OS, new PV drivers, etc. We also provide a Quick Start Package (http://dornerworks.com/xen/xen-quick-start) to help train others on Xen, virtualization, and specific platforms. This is a good option for both those that are just getting their feet wet and those that want to know all the nitty-gritty details.

What are some of the major changes you see with virtualization?
We see virtualization becoming more and more of a necessity in the embedded world. As the complexity of processors increases, the difficulty of utilizing them increases. Processors, like the Zynq UltraScale+ MPSoC, that have a Quad-Core ARM Cortex-A53, a Dual-Core ARM Cortex-R5, and an FPGA in a single chip, can be difficult to manage. Virtualization provides a means to isolate out various pieces in a more manageable and effective way. Not only does the Xen Project Hypervisor help manage complexity, but it also can reduce size, weight, and power (SWaP), provide redundancy, address obsolescence of legacy systems, and more.

However, while the temptation is to use virtualization to create a single integrated platform for all computation, this approach could create a single point of failure unless it is mitigated by system wide redundancies. In these applications, Xen Project technologies can be used to provide an embedded “cloud,” which provides the reliability required by the application with a large measure of integration. This approach is both familiar and different in embedded applications, which frequently use both hardware and software to provide both isolation and redundancy, but have traditionally leaned more on hardware based solutions.

What advice would you give someone considering joining the Xen Project?
Just jump in and get involved. Go to the conferences, meet people, submit patches, review patches, ask questions, and enjoy yourself. It is a great community that is friendly, open, and has a lot of people with similar goals. They want to help each other and improve the technologies we are all utilizing. I have personally had a blast at the Developer’s Summit and look forward to going to more.

What excites you most about the future of Xen?
I am excited to see hardware become more virtualization friendly. When Xen can utilize these features, the overhead added to the system can be decreased even further than it already has been. This will help make the Xen Project Hypervisor an even more attractive solution in the embedded space.

Embedded hypervisors have been around for a long time, but with the increasingly complex SoCs being produced by chip vendors and the industry drive towards system integration, the number of deployed hypervisor based embedded systems continues to increase. While it has taken longer than we thought when we first joined the Xen Project community, we can see the fruits of these efforts starting to pay off.  We are excited to be a part of the many Xen Project contributors putting Xen in systems quite different from the cloud, utilizing the same underlying technologies in order to provide the security and reliability we have become accustomed to in cloud applications to embedded ones.

Additionally when we first started working with the Xen Project there was not much talk about the safety certification of Xen, but with the increasing interest of the automotive industry in hypervisors, we are seeing a lot of discussion and progress on this front. There is still a long way to go, but at least the will is currently there.

*If you want to stay in the know around embedded virtualization and Xen, sign up for DornerWork’s weekly newsletter here

Xen Project Spectre / Meltdown FAQ (Jan 22 Update)

This document has been updated since it was published! Updates are marked in blue.

On January 3rd, 2018, Google’s Project Zero announced several information leak vulnerabilities affecting all modern superscalar processors. Details can be found on their blog, and in the Xen Project Advisory 254. To help our users understand the impact and our next steps forward, we put together the following FAQ. We divided the FAQ into several sections to make it easier to consume:

  • General considerations affecting all 3 vulnerabilities
  • “Rogue Data Load” (aka SP3, “Variant 3”, Meltdown, CVE-2017-5754)
  • “Branch Target Injection” (aka SP2, “Variant 2”, Spectre CVE-2017-5715)
  • “Bounds-check bypass” (aka SP1, “Variant 1”, Spectre CVE-2017-5753)

The project has been developing patches in order of exploitability. Our initial focus was on fixes for Meltdown, then on fixes for Spectre Variant 2, and finally on Variant 1.

Generally in the context of Xen based systems, there are many different considerations that have gone into our strategy, such as

  • Can a guest (user or kernel space) attack the hypervisor using Meltdown or Spectre?
  • Can a guest (user or kernel space) attack another guest (user or kernel space) when running in a Xen VM?

Note that impact and mitigations are specific to CPU architectures (and in some cases models) and may also differ depending on virtualization mode. The below FAQ tries to lay this out clearly, but if you have any questions, please reply to this email thread.

Note that we will update or re-issue the FAQ on this blog as new information surfaces.

1) General Questions related to all 3 vulnerabilities

1.1) Is there any risk of privilege escalation?

Meltdown and Spectre are, by themselves, only information leaks. There is no suggestion that speculative execution can be used to modify memory or cause the system to do anything it might not have done already.

1.2) Where can I find more information?

We will update this blog post and Advisory 254 as new information becomes available. Updates will also be published on the xen-announce@ mailing list.

1.3) Where can I ask questions?

This blog post, has been posted in text form on the xen-devel@ mailing list. If you have questions or improvement suggestions, please reply to this email thread.

1.4) Where does development of mitigations happen?

As information related to Meltdown and Spectre is now public, development will continue in public on xen-devel@ and patches will be posted and attached to Advisory 254 as they become available.

2) SP3, “Variant 3”, Meltdown, CVE-2017-5754

2.1) Is Xen impacted by Meltdown (“Variant 3”)?

Only Intel processors are impacted by Meltdown (referred to as SP3 in Advisory 254). On Intel processors, only 64-bit PV mode guests can attack Xen. Guests running in 32-bit PV mode, HVM mode, and PVH mode cannot attack the hypervisor using SP3.

Note that in general, some ARM processors are impacted by Meltdown (see https://developer.arm.com/support/security-update): however these cannot be exploited on Xen.

Guest Type

Is a user space attack from a guest to Xen possible?

Is a kernel space attack from a guest to Xen possible?

Available Mitigations

32 bit PV

No

No

N/A

64 bit PV

Yes

Yes

Several with different trade-offs
See Question 2.2

HVM

No

No

N/A

PVH

No

No

N/A

ARM [1]

No

No

N/A

Notes:
[1] ARM’s security update refers to a subvariant of Meltdown called “Variant 3a”. The impact analysis of this variant is not yet fully complete, but we believe that no sensitive data can be leaked to exploit Xen.

2.2) Are there any patches available for Meltdown (“Variant 3”)?

The project has published five different mitigations with Advisory 254 following different mitigation strategies for Meltdown. Two strategies involve switching from PV guests to PVH or HVM guests. The others require application of patches as outlined in Advisory 254:

  • Vixen: The basic principle is to run PV guests (which can read all of host memory due to Meltdown) as HVM guests (which cannot read memory due to Meltdown) using a hypervisor shim.
  • Comet: The basic principle is to run PV guests (which can read all of host memory due to the hardware bugs) as PVH guests (which cannot read memory due to Meltdown) using a hypervisor shim.
  • PTI or Xen PTI stage-1: This solution implements Page Table Isolation (PTI) for Xen.

Each strategy has different trade-offs and will work well for some use-cases, but not others. A high-level comparison of the different trade-offs for each mitigation, including information about code and documentation can be found in Advisory 254 (under “SP3 MITIGATION OPTIONS SUMMARY TABLE FOR 64-bit X86 PV GUESTS”). Please make sure you carefully read this section and the README files in the advisory.

2.3) How are Xen Guests impacted by Meltdown (“Variant 3”)?

In 32-bit PV mode, HVM mode, and PVH mode, guest user spaces can attack guest kernels using SP3; so updating guest kernels is advisable. Interestingly, guest kernels running in 64-bit PV mode are not vulnerable to attack using SP3, but attacks on user and kernel spaces of other guests are possible.

Guest Type

Is a user space attack on the guest kernel possible (when running in a Xen VM)?

Is a user space attack on other guests possible (when running in a Xen VM)?

Is a kernel space attack on other guests possible (when running in a Xen VM)?

32 bit PV

Yes [1]

No

No

64 bit PV

No [2]

Yes [3]

Yes [3]

HVM

Yes [1]

No

No

PVH

Yes [1]

No

No

ARM

Yes [1]

No

No

Mitigations and notes:
[1] Can be mitigated by the Linux KPTI patch and similar patches for other operating systems
[2] Although, a direct user space attack on the kernel is not possible, user space can indirectly be exploited via [3]. When Vixen and Comet are deployed, all guest memory is mapped by the “shim,” which is itself vulnerable to Meltdown. The Xen PTI patches protect both the hypervisor and the guest kernel from attacks from the guest user (without need for additional guest kernel patches). Note that KPTI is automatically disabled when running in 64 bit PV guests: thus running Xen PTI together with KPTI should not have any adverse effects.
[3] Mitigated by stage-1 Xen PTI

2.4) What is the long-term plan for Meltdown (“Variant 3”)?

Longer term, we will merge Vixen with Comet and release in suitable Xen Point releases with the codename Rudolph. In addition, we will improve PTI. We will likely backport and release PTI in suitable Xen point releases.

Note that Vixen and Comet will not be released in Xen point releases, but only through Advisory 254.

2.5) Does Xen have any equivalent to Linux’s KPTI series?

Linux’s KPTI series is designed to address SP3 only. For Xen guests, only 64-bit PV guests are affected by SP3. We have released a PTI (sometimes called Xen PTI or XPTI) series, which we will continue to improve over the coming weeks.

3) SP2, “Variant 2”, Spectre, CVE-2017-5715

3.1) Is Xen impacted by Spectre (“Variant 2”)?

Both Intel and AMD CPUs are vulnerable to Spectre (both variants). Vulnerability of ARM processors to Spectre (both variants) varies by model and manufacturer.

Guest Type

Is a user space attack from a guest to Xen possible?

Is a kernel space attack from a guest to Xen possible?

Available Mitigations

x86

Yes

Yes

See Question 3.4.1

ARM 32 [1]

Yes

Yes

See Question 3.4.2

ARM 64 [1]

Yes

Yes

Mitigations and notes:
[1] ARM has information on affected models on the following website: https://developer.arm.com/support/security-update. According to Cavium Thunder X1 is not vulnerable to Spectre (both variants).

3.2) How are Xen Guests impacted by Spectre (“Variant 2”)?

Both Intel and AMD CPUs are vulnerable to Spectre (both variants). Vulnerability of ARM processors to Spectre (both variants) varies by model and manufacturer.

Guest Type

Is a user space attack on other user processes or the guest kernel within the same guest possible (when running in a Xen VM)?

Is a user space attack on other guests possible (when running in a Xen VM)?

Is a kernel space attack on other guests possible (when running in a Xen VM)?

x86

Yes [2]

Yes [3]

Yes [3]

ARM 32 [1]

Yes [2]

Yes [4]

Yes [4]

ARM 64 [1]

Yes [2]

Yes [5]

Yes [5]

Mitigations and notes:
[1] ARM has information on affected models on the following website: https://developer.arm.com/support/security-update. According to Cavium Thunder X1 is not vulnerable to Spectre (both variants).
[2] Mitigated by retpoline or firmware based Indirect Branch Control mitigations in guest operating systems (see here for Linux Kernel mitigations)
[3] Mitigated by “Intel and AMD CPUs” approach as outlined in question 3.4.1
[4] Mitigated by “Affected ARM CPUs” (64 bit) approach as outlined in question 3.4.2
[5] Mitigated by “Affected ARM CPUs” (32 bit) approach as outlined in question 3.4.2

3.3) Are mitigations for Spectre possible (“Variant 2”)?

SP2 can be mitigated in two ways, both of which essentially prevent speculative execution of indirect branches. The first is to flush the branch prediction logic on entry into the hypervisor. This requires microcode updates, which Intel and AMD are in the process of preparing, as well as patches to the hypervisor which are also in process and should be available soon. On ARM, firmware updates are required (see here).

The second is to do indirect jumps in a way that is not subject to speculative execution (this approach is called Retpoline). This requires the hypervisor to be recompiled with a compiler that contains special new features. These new compiler features are also in the process of being prepared for both GCC (see here and here) and clang, and should be available soon.

3.4) Are there any patches available for Spectre (“Variant 2”)?

3.4.1) Intel and AMD CPUs:

We have developed prototype patches for x86 CPUs. These patches depend on firmware updates. Our prototype patches were developed against pre-released versions of MSR specifications and were modified to comply with published MSR specifications (see here). This may require changes to our patches. There have also been reports of issues with some published firmware updates (see here and here) leading to frequent reboots of systems where these have been deployed. We are currently evaluating the situation to verify whether Xen based systems with mitigations are affected.

Variant 2 mitigations have been published via Advisory 254. More information on ongoing development can be found on relevant xen-devel@ discussions which are linked to from here.

3.4.2) Affected ARM CPUs:

A framework to mitigate Spectre Variant 2 has been developed (for 64 bit and 32 bit). CPU vendors, will be able to add support for specific CPUs to the framework.

The framework and vendor specific mitigations has been published via Advisory 254 (including support for ARM CPUs). More information on ongoing development can be found on relevant xen-devel@ discussions which are linked to from here.

4) SP1, “Variant 1”, Spectre, CVE-2017-5753

4.1) Is Xen impacted by Spectre (“Variant 1”)?

Both Intel and AMD CPUs are vulnerable to Spectre (both variants). Vulnerability of ARM processors to Spectre (both variants) varies by model and manufacturer.

Guest Type

Is a user space attack from a guest to Xen possible?

Is a kernel space attack from a guest to Xen possible?

Available Mitigations

x86

Yes

Yes

See Question 4.3

ARM [1]

Yes

Yes

Mitigations and notes:
[1] ARM has information on affected models on the following website: https://developer.arm.com/support/security-update. According to Cavium Thunder X1 is not vulnerable to Spectre (both variants).

4.2) How are Xen Guests impacted by Spectre (“Variant 1”)?

Both Intel and AMD CPUs are vulnerable to Spectre (both variants). Vulnerability of ARM processors to Spectre (both variants) varies by model and manufacturer.

Guest Type

Is a user space attack on other user processes or the guest kernel within the same guest possible (when running in a Xen VM)?

Is a user space attack on other guests possible (when running in a Xen VM)?

Is a kernel space attack on other guests possible (when running in a Xen VM)?

x86

Yes [2]

Yes [3]

Yes [3]

ARM [1]

Yes [2]

Yes [3]

Yes [3]

Mitigations and notes:
[1] ARM has information on affected models on the following website: https://developer.arm.com/support/security-update. According to Cavium Thunder X1 is not vulnerable to Spectre (both variants).
[2] Please refer to guest operating specific mitigations (see here for Linux Kernel mitigations)
[3] See question 4.3

4.3) Are mitigations for Spectre possible (“Variant 1”)?

Spectre Variant 1 is much more difficult to mitigate. We have some ideas we’re exploring, but they’re still at the design stage at this point.

Xen Project Spectre/Meltdown FAQ

Updated to v3 on Dec 12th!

Google’s Project Zero announced several information leak vulnerabilities affecting all modern superscalar processors. Details can be found on their blog, and in the Xen Project Advisory 254. To help our users understand the impact and our next steps forward, we put together the following FAQ.

Note that we will update the FAQ as new information surfaces.

Changes since the initial publication:

  • v3: Added information related to Comet mitigation for Variant 3 (Meltdown) – for now Xen 4.10 only
  • v2: Added information related to Vixen mitigation for Variant 3 (Meltdown) – see Are there any patches for the vulnerability?
  • v2: Replaced SPx with Variant x to be in line with the terminology used elsewhere vulnerability?

Is Xen impacted by Meltdown and Spectre?

There are two angles to consider for this question:

  • Can an untrusted guest attack the hypervisor using Meltdown or Spectre?
  • Can a guest user-space program attack a guest kernel using Meltdown or Spectre?

Systems running Xen, like all operating systems and hypervisors, are potentially affected by Spectre (referred to as Variant 1 and 2 in Advisory 254). For Arm Processors information, you can find which processors are impacted here.  In general, both the hypervisor and a guest kernel are vulnerable to attack via Variant 1 and 2.

Only Intel processors are impacted by Meltdown (referred to as Variant 3 in Advisory 254). On Intel processors, only 64-bit PV mode guests can attack Xen using Variant 3. Guests running in 32-bit PV mode, HVM mode, and PVH mode (both v1 and v2) cannot attack the hypervisor using Variant 3. However, in 32-bit PV mode, HVM mode, and PVH mode (both v1 and v2), guest userspaces can attack guest kernels using Variant 3; so updating guest kernels is advisable.

Interestingly, guest kernels running in 64-bit PV mode are not vulnerable to attack using Variant 3, because 64-bit PV guests already run in a KPTI-like mode.

However, keep in mind that a successful attack on the hypervisor can still be used to recover information about the same guest from physical memory.

Is there any risk of privilege escalation?

Meltdown and Spectre are, by themselves, only information leaks. There is no suggestion that speculative execution can be used to modify memory or cause a system to do anything it might not have done already.

Where can I find more information?

We will update this blog post and Advisory 254 as new information becomes available. Updates will also be published on xen-announce@.

We will also maintain a technical FAQ on our wiki for answers to more detailed technical questions that emerge on xen-devel@ (and in particular this e-mail thread) and other communication channels.

Are there any patches for the vulnerability?

We have published a mitigation for Meltdown on Intel CPUs: please refer to Advisory 254. The published solutions are labelled Vixen and Comet (also see README.which-shim). Alternative solutions are being worked on.

A Mitigation for Variant 2/CVE-2017-5715 is available on the xen-devel@ mailing list, but has not yet undergone rigorous enough review to be published as an official patch.

As information related to Meltdown and Spectre is now public, development will continue in public on xen-devel@ and patches will be posted and attached to Advisory 254 as they become available in the next few days.

Can Variant 1/Variant 2 be fixed at all? What plans are there to mitigate them?

Variant 2 can be mitigated in two ways, both of which essentially prevent speculative execution of indirect branches. The first is to flush the branch prediction logic on entry into the hypervisor. This requires microcode updates, which Intel and AMD are in the process of preparing, as well as patches to the hypervisor which are also in process and should be available soon.

The second is to do indirect jumps in a way which is not subject to speculative execution. This requires the hypervisor to be recompiled with a compiler that contains special new features. These new compiler features are also in the process of being prepared for both gcc and clang, and should be available soon.

Variant 1 is much more difficult to mitigate. We have some ideas we’re exploring, but they’re still at the design stage at this point.

Does Xen have any equivalent to Linux’s KPTI series?

Linux’s KPTI series is designed to address Variant 3 only. For Xen guests, only 64-bit PV guests are able to execute a Variant 3 attack against the hypervisor. A KPTI-like approach was explored initially, but required significant ABI changes.  Instead we’ve decided to go with an alternate approach, which is less disruptive and less complex to implement. The chosen approach runs PV guests in a PVH container, which ensures that PV guests continue to behave as before, while providing the isolation that protects the hypervisor from Variant 3. This works well for Xen 4.8 to Xen 4.10, which is currently our priority.

For Xen 4.6 and 4.7, we are evaluating several options, but we have not yet finalized the best solution.

Devicemodel stub domains run in PV mode, so is it still more safe to run device models in a stub domain than in domain 0?

The short answer is, yes, it is still safer to run stub domains than otherwise.

If an attacker can gain control of the device model running in a stub domain, it can indeed attempt to use these processor vulnerabilities to read information from Xen.

However, if an attacker can gain control of a device model running in domain 0 without deprivileging, the attacker can gain control of the entire system.  Even with qemu deprivileging, the qemu process may be able to execute speculative execution attacks against the hypervisor.

So although XSA-254 does affect device model stub domains, they are still safer than not running with a stub domain.

What is the Xen Project’s plan going forward?

The Xen Project is working on finalizing solutions for Variant 3 and Variant 2 and evaluating options for Variant 1. If you would like to stay abreast on our progress, please sign up to xen-announce@. We will update this FAQ as soon as we have more news and updated information. Answers to more detailed technical questions will be maintained in a technical FAQ on our wiki. Thank you for your patience.

How can I ask further questions?

Please respond to this e-mail thread on xen-devel@ or xen-users@.