Re: [PATCH]: Fix MSR_K7_EVNTSEL{0,3} for SVM

Chris Lalancette
2008-04-28T14:17:37Z

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Joerg Roedel wrote:
> We already discussed the emulation of the performance counter registers
> in the past. The conclusion is, that we loose live migration with that
> emulation because performance monitoring is implemented differently
> between AMD and Intel systems. Maybe discarding _any_ writes to
> the performance counter MSRs will fix the guest crash. What we should
> not do is injecting GPF on zero writes to the MSRs because that will
> break Windows XP 64bit installation.

OK, yeah, I read some of those threads over the weekend. It is a larger
problem than just this crash, and probably one to be solved later.
I have attached a patch which just discards all writes, as you suggested;
this allows my RHEL-4 guest to successfully boot, although obviously the NMI
watchdog which it thinks it set up is not actually working. Joerg, can you test
this on your 64-bit Windows XP guest to make sure it is still working?

Thanks,
Chris Lalancette

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diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 89e0be2..6130704 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1284,16 +1284,18 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, unsigned ecx, u64 data)
case MSR_K7_EVNTSEL1:
case MSR_K7_EVNTSEL2:
case MSR_K7_EVNTSEL3:
+ case MSR_K7_PERFCTR0:
+ case MSR_K7_PERFCTR1:
+ case MSR_K7_PERFCTR2:
+ case MSR_K7_PERFCTR3:
/*
- * only support writing 0 to the performance counters for now
- * to make Windows happy. Should be replaced by a real
- * performance counter emulation later.
+ * Just discard all writes to the performance counters; this
+ * should keep both older linux and windows 64-bit guests
+ * happy
*/
- if (data != 0)
- goto unhandled;
+
break;
default:
- unhandled:
return kvm_set_msr_common(vcpu, ecx, data);
}
return 0;

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