On 2011-06-13 10:45, Avi Kivity wrote:
> On 06/11/2011 12:23 PM, Jan Kiszka wrote:
>> From: Jan Kiszka<address@hidden>
>>
>> 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?
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.