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Re: [Xen-devel] [PATCH v5][XSA-97] x86/paging: make log-dirty operations preemptible

>>> On 15.09.14 at 15:56, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 15/09/2014 13:54, Jan Beulich wrote:
>>>>> On 15.09.14 at 09:50, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> It is indeed migration v2, which is necessary in XenServer given our
>>> recent switch from 32bit dom0 to 64bit.  The counts are only used for
>>> logging, and debugging purposes; all movement of pages is based off the
>>> bits in the bitmap alone.  In particular, the dirty count is used as a
>>> basis of the statistics for the present iteration of migration.  While
>>> getting it wrong is not the end of the world, it would certainly be
>>> preferable for the count to be accurate.
>>> As for the memory corruption, XenRT usually tests pairs of VMs at a time
>>> (32 and 64bit variants) and all operations as back-to-back as possible.
>>> Therefore, it is highly likely that a continued operation on one domain
>>> intersects with other paging operations on another.
>> But there's nothing I can see where domains would have a way
>> of getting mismatched. It is in particular this one
>> (XEN) [ 7832.953068] mm.c:827:d0v0 pg_owner 100 l1e_owner 100, but 
> real_pg_owner 99
>> which puzzles me: Assuming Dom99 was the original one, how
>> would Dom100 get hold of any of Dom99's pages (IOW why would
>> Dom0 map one of Dom99's pages into Dom100)? The patch doesn't
>> alter any of the page refcounting after all. Nor does your v2
>> migration series I would think.
> In this case, dom99 was migrating to dom100.  The failure was part of 
> verifying dom100v0's cr3 at the point of loading vcpu state, so Xen was 
> in the process of pinning pagetables.
> There were no errors on pagetable normalisation, so dom99's PTEs were 
> all correct, and there were no errors restoring any of dom100's memory, 
> so Xen fully allocated frames for dom100's memory during 
> populate_phymap() hypercalls.
> During pagetable normalisation, dom99's pfns in the stream are converted 
> to dom100's mfns as per the newly created p2m from the 
> populate_physmap() allocations.  Then during dom100's cr3 validation, it 
> finds a dom99 PTE and complains.
> Therefore, a frame Xen handed back to the toolstack as part of 
> allocating dom100's memory still belonged to dom99.

Or on the saving side some page table(s) didn't get normalized at
all (in which case there necessarily also were no errors detected
with that). Not being marked as page table(s) would then also lead
to not getting converted back to machine representation on restore,
resulting in a reference to a page belonging to the old domain.

But together with the memory corruption you mentioned seen in
HVM guests all of the above may just be secondary effects.


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