Il 09/09/2013 11:03, Gleb Natapov ha scritto: > On Mon, Sep 09, 2013 at 10:31:15AM +0200, Paolo Bonzini wrote: >> Il 08/09/2013 13:40, Gleb Natapov ha scritto: >>> On Thu, Sep 05, 2013 at 03:06:21PM +0200, Paolo Bonzini wrote: >>>> On KVM, the KVM_SET_XSAVE would be executed with a 0 xstate_bv, >>>> and not restore anything. >>>> >>> XRSTOR restores FP/SSE state to reset state if no bits are set in >>> xstate_bv. This is what should happen on reset, no? >> >> Yes. The problem happens on the migration destination when XSAVE data is >> not transmitted. FP/SSE data is transmitted and must be restored, but >> xstate_bv is zero and KVM_SET_XSAVE restores FP/SSE state to reset >> state. The vcpu then loses the values that were set in the migration data. >> >>>> Since FP and SSE data are always valid, set them in xstate_bv at reset >>>> time. In fact, that value is the same that KVM_GET_XSAVE returns on >>>> pre-XSAVE hosts. >>> It is needed for migration between non xsave host to xsave host. >> >> Yes, and this patch does the same for migration between non-XSAVE QEMU >> and XSAVE QEMU. >> > Can such migration happen? The commit that added xsave support > (f1665b21f16c5dc0ac37de60233a4975aff31193) changed vmstate version id. Yes, old->new migration can happen. New->old of course cannot. >> In fact, another bug is that kvm_vcpu_ioctl_x86_set_xsave ignores >> xstate_bv when XSAVE is not available. Instead, it should reset the >> FXSAVE data to processor-reset values (except for MXCSR which always >> comes from XRSTOR data), i.e. to all-zeros except for the x87 control >> and tag words. It should also check reserved bits of MXCSR. > > I do not see why. Because otherwise it behaves in a subtly different manner for XSAVE and non-XSAVE hosts. >> Yes. QEMU unmarshals information from the XSAVE region and back, so it >> cannot support MPX or AVX-512 yet (even if KVM were). Separate bug, though. >> > IMO this is the main issue here, not separate bug. If we gonna let guest > use CPU state QEMU does not support we gonna have a bad time. We cannot force the guest not to use a feature; all we can do is hide the CPUID bits so that a well-behaved guest will not use it. QEMU does hide CPUID bits for non-supported XSAVE states, except for "-cpu host". So this will not be a problem except with "-cpu host". Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
