Re: [PATCH][uq/master] kvm: x86: Save/restore FPU OP, IP and DP

Avi Kivity
2011-06-15T09:11:05Z

On 06/14/2011 09:10 AM, Jan Kiszka wrote:
> On 2011-06-13 10:45, Avi Kivity wrote:
> > On 06/11/2011 12:23 PM, Jan Kiszka wrote:
> >> From: Jan Kiszka<jan.kiszka@siemens.com>
> >>
> >> These FPU states are properly maintained by KVM but not yet by TCG. So
> >> far we unconditionally set them to 0 in the guest which may cause
> >> state corruptions - not only during migration.
> >>
> >>
> >> -#define CPU_SAVE_VERSION 12
> >> +#define CPU_SAVE_VERSION 13
> >>
> >
> > Incrementing the version number seems excessive - I can't imagine a
> > real-life guest will break due to fp pointer corruption
> >
> > However, I don't think we have a mechanism for optional state. We
> > discussed this during the 18th VMState Subsection Symposium and IIRC
> > agreed to re-raise the issue when we encountered it, which appears to be
> > now.
> >
>
> Whatever we invent, it has to be backported as well to allow that
> infamous traveling back in time, migrating VMs from newer to older versions.
>
> Would that backporting be simpler if we used an unconditional subsection
> for the additional states?

Thinking about it, a conditional subsection would work fine. Most
threads will never see an fpu error, and are all initialized to a clean
slate.

SDM 1 8.1.9.1 says:

> 8.1.9.1 Fopcode Compatibility Sub-mode
> Beginning with the Pentium 4 and Intel Xeon processors, the IA-32
> architecture
> provides program control over the storing of the last instruction
> opcode (sometimes
> referred to as the fopcode). Here, bit 2 of the IA32_MISC_ENABLE MSR
> enables (set)
> or disables (clear) the fopcode compatibility mode.
> If FOP code compatibility mode is enabled, the FOP is defined as it
> has always been
> in previous IA32 implementations (always defined as the FOP of the
> last non-trans-
> parent FP instruction executed before a FSAVE/FSTENV/FXSAVE). If FOP code
> compatibility mode is disabled (default), FOP is only valid if the
> last non-transparent
> FP instruction executed before a FSAVE/FSTENV/FXSAVE had an unmasked
> exception.

So fopcode will usually be clear.

--
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