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

Jan Kiszka
2011-06-15T10:21:45Z

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On 2011-06-15 11:10, Avi Kivity wrote:
> 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.
>

OK. So if bit 2 of IA32_MISC_ENABLE MSR, we must save that fields. But
if it's off, how to test for that other condition "last non-transparent
FP instruction ... had an unmasked exception" from the host?

Jan

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