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Re: [Qemu-devel] [PATCH 1/3] xen: fix quad word bufioreq handling

From: Jan Beulich
Subject: Re: [Qemu-devel] [PATCH 1/3] xen: fix quad word bufioreq handling
Date: Wed, 23 Nov 2016 03:36:00 -0700

>>> On 23.11.16 at 10:48, <address@hidden> wrote:
>> From: Jan Beulich [mailto:address@hidden
>> Sent: 23 November 2016 09:24
>> We should not consume the second slot if it didn't get written yet.
>> Normal writers - i.e. Xen - would not update write_pointer between the
>> two writes, but the page may get fiddled with by the guest itself, and
>> we're better off entering an infinite loop in that case.
> Xen would never put QEMU in this situation and the guest can't actually 
> modify the page whilst it's in use, since activation of the IOREQ server 
> removes the page from the guest's p2m so the premise of the patch is not 
> correct.

Is that the case even for pre-ioreq-server Xen versions? The issue
here was reported together with what became XSA-197, and it's
not been assigned its own XSA just because there are other ways
for a guest to place high load on its qemu process (and there are
ways to deal with such high load situations).


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