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.kis...@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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