Re: [Xen-devel] [PATCH 2/2] x86emul: adjust MOVSXD source operand handling

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.10.2019 21:44, Andrew Cooper wrote:
> In this example, hardware can the emulator can disagree by using a
> different access width.
> 
> I've been experimenting with my Rome system, and an emulator hardcoded
> to use 2-byte accesses.  After some investigation, the livelock only
> occurs for access-rights faults.  Translation faults get identified as
> not a shadow fault, and bounced back to the guest.
> 
> Shadow guests can use PKRU, so can generate an access fault by marking
> the page at 0x2000 as no-access, so I think that in principle, this
> change will result in a new latent livelock case, but I can't actually
> confirm it.

I think I see what you mean, but then I don't see how this is an
argument against the patch in its current shape: It actually
reduces the cases of disagreement between hardware and emulator.
One possibility to make a further step in that direction would
be to make behavior dependent upon the underlying hardware's
vendor, rather than the one the guest sees.

Jan

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