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Re: [Xen-devel] fsincos emulation on AMD CPUs


  • To: Jan Beulich <JBeulich@xxxxxxxx>
  • From: Keir Fraser <keir@xxxxxxx>
  • Date: Thu, 15 Dec 2011 13:13:58 +0000
  • Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
  • Delivery-date: Thu, 15 Dec 2011 13:14:27 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: Acy7K2dr1qaUWoglu0O9DDUjmy0qpA==
  • Thread-topic: [Xen-devel] fsincos emulation on AMD CPUs

On 15/12/2011 13:08, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:

>>>> On 15.12.11 at 13:33, Keir Fraser <keir@xxxxxxx> wrote:
>> More detail: the full patch is ugly and hard to test all cases. And there's
>> no practical scenario where we want to emulate FSINCOS on AMD -- we don't
>> emulate realmode on AMD, FSINCOS on a shadowed page certainly indicates that
>> we should unshadow the page, FSINCOS on MMIO is mad or malicious.
> 
> Those latter two cases can't really happen, as fsincos has no memory
> operand.

Possibly if the instruction itself was in a recycled page-table page? Or in
an MMIO page, or the malicious race that Paolo described --- definitely
malicious either way.

Anyhow the short answer is we never want to emulate it on AMD. :-)

 -- Keir



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