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envelope-from=william.roche@oracle.com; helo=mx0b-00069f02.pphosted.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --------------HwKxFBMtz3PUrQYhSERwJR0D Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 11/8/23 22:45, Peter Xu wrote: > On Mon, Nov 06, 2023 at 10:38:14PM +0100, William Roche wrote: >> But it implies a lot of other changes: >>     - The source has to flag the error pages to indicate a poison >>       (new flag in the exchange protocole) >>     - The destination has to be able to deal with the new protocole > IIUC these two can be simply implemented by migrating hwpoison_page_list > over to dest. You need to have a compat bit for doing this, ignoring the > list on old machine types, because old QEMUs will not recognize this vmsd. > > QEMU should even support migrating a list object in VMSD, feel free to have > a look at VMSTATE_QLIST_V(). This is another area that I'll need to learn about. >>     - The destination has to be able to mark the pages as poisoned >>       (authorized to use userfaultfd) > Note: userfaultfd is actually available without any privilege if to use > UFFDIO_POISON only, as long as to open the uffd (either via syscall or > /dev/userfaultfd) using UFFD_FLAG_USER_ONLY. > > A trick is we can register with UFFD_WP mode (not MISSING; because when a > kernel accesses a missing page it'll cause SIGBUS then with USER_ONLY), > then inject whatever POISON we want. As long as UFFDIO_WRITEPROTECT is not > invoked, UFFD_WP does nothing (unlike MISSING). > >>     - So both source and destination have to be upgraded (of course >>       qemu but also an appropriate kernel version providing >>       UFFDIO_POISON on the destination) > True. Unfortunately this is not avoidable. > >>     - we may need to be able to negotiate a fall back solution >>     - an indication of the method to use could belong to the >>       migration capabilities and parameters > For above two points: it's a common issue with migration compatibility. As > long as you can provide above VMSD to migrate hwpoison_page_list, marking > all old QEMU machine types skipping that, then it should just work. > > You can have a closer look at anything in hw_compat_* as an example. Yes, I'll do that. >>     - etc... > I think you did summarize mostly all the points I can think of; is there > really anything more? :) Probably some work to select the poison migration method (allowing a migration transforming poison into zeros as a fall back method if the poison migration itself with UFFDIO_POISON can't be used, or not) for example. > It'll be great if you can, or plan to, fix that for good. Thanks for the offer ;) I'd really like to implement that, but I currently have another pressing issue to work on. I should be back on this topic within a few months. I'm now waiting for some feedback from the ARM architecture reviewer(s). Thanks a lot for all your suggestions. --------------HwKxFBMtz3PUrQYhSERwJR0D Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: 8bit
On 11/8/23 22:45, Peter Xu wrote:
On Mon, Nov 06, 2023 at 10:38:14PM +0100, William Roche wrote:
But it implies a lot of other changes:
    - The source has to flag the error pages to indicate a poison
      (new flag in the exchange protocole)
    - The destination has to be able to deal with the new protocole

IIUC these two can be simply implemented by migrating hwpoison_page_list
over to dest.  You need to have a compat bit for doing this, ignoring the
list on old machine types, because old QEMUs will not recognize this vmsd.

QEMU should even support migrating a list object in VMSD, feel free to have
a look at VMSTATE_QLIST_V().

This is another area that I'll need to learn about.

    - The destination has to be able to mark the pages as poisoned
      (authorized to use userfaultfd)

Note: userfaultfd is actually available without any privilege if to use
UFFDIO_POISON only, as long as to open the uffd (either via syscall or
/dev/userfaultfd) using UFFD_FLAG_USER_ONLY.

A trick is we can register with UFFD_WP mode (not MISSING; because when a
kernel accesses a missing page it'll cause SIGBUS then with USER_ONLY),
then inject whatever POISON we want.  As long as UFFDIO_WRITEPROTECT is not
invoked, UFFD_WP does nothing (unlike MISSING).


    - So both source and destination have to be upgraded (of course
      qemu but also an appropriate kernel version providing
      UFFDIO_POISON on the destination)

True.  Unfortunately this is not avoidable.


    - we may need to be able to negotiate a fall back solution
    - an indication of the method to use could belong to the
      migration capabilities and parameters

For above two points: it's a common issue with migration compatibility.  As
long as you can provide above VMSD to migrate hwpoison_page_list, marking
all old QEMU machine types skipping that, then it should just work.

You can have a closer look at anything in hw_compat_* as an example.

Yes, I'll do that.


      

        
    - etc...


      

      
I think you did summarize mostly all the points I can think of; is there
really anything more? :)

    

    
Probably some work to select the poison
        migration method (allowing a

        migration transforming poison into zeros as a fall back method
        if the

        poison migration itself with UFFDIO_POISON can't be
        used, or not) for

        example.

    

    

      
It'll be great if you can, or plan to, fix that for good.


    

    
Thanks for the offer ;)

        I'd really like to implement that, but I currently have another
        pressing

        issue to work on. I should be back on this topic within a few
        months.

    
I'm now waiting for some feedback from the
        ARM architecture reviewer(s).

    
Thanks a lot for all your suggestions.

      

  


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