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

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

On 12/15/2011 09:38 AM, Jan Beulich wrote:
> All,
>
> in the light of erratum #573 I'm wondering if we need to tweak or
> conditionally suppress fsincos emulation. The question is whether there
> is any possibility for getting the emulator to hit this instruction on AMD
> (as no real mode emulation ought to be taking place there), i.e.
> whether there are places where emulation gets continued eagerly
> in anticipation of the need for emulation on a nearby instruction.
This can happen with PAE + shadow pagetables.

There's also the case when a user process issues an instruction to an 
MMIO region, and another thread replaces the instruction with another 
(fsincos in this case), racing with the emulator until the emulator sees 
fsincos instead of the MMIO instruction.
If you really cared, perhaps fsincos can be replaced by this sequence in 
the emulator:
                    ; x
    fld   %st       ; x x
    fsin            ; x sin(x)
    fxch  %st(1)    ; sin(x) x
    fcos            ; sin(x) cos(x)

Paolo


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