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Re: [Xen-devel] [PATCH 0/2] MMIO emulation fixes


  • To: Olaf Hering <olaf@xxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Wed, 29 Aug 2018 11:53:05 +0100
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  • Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Paul Durrant <paul.durrant@xxxxxxxxxx>, george.dunlap@xxxxxxxxxx
  • Delivery-date: Wed, 29 Aug 2018 10:53:20 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 29/08/18 11:36, Olaf Hering wrote:
> On Mon, Aug 13, Jan Beulich wrote:
>
>> And hence the consideration of mapping in an all zeros page
>> instead. This is because of the way __hvmemul_read() /
>> __hvm_copy() work: The latter doesn't tell its caller how many
>> bytes it was able to read, and hence the former considers the
>> entire range MMIO (and forwards the request for emulation).
>> Of course all of this is an issue only because
>> hvmemul_virtual_to_linear() sees no need to split the request
>> at the page boundary, due to the balloon driver having left in
>> place the mapping of the ballooned out page.
> Should perhaps __hvm_copy detect the fault and copy 0xf for the
> unavailable page into 'buf', and finally return success?
>
> Clearly something must be done at the Xen level.

This is first and formost a Linux bug.  No amount of fixing Xen is going
to alter that.

Architecturally speaking, handing #MC back is probably the closest we
can get to sensible behaviour, but it is still a bug that Linux is
touching the ballooned out page in the first place.

~Andrew

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