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

Chris Lalancette
2008-04-23T18:23:14Z

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Avi, Joerg,
While trying to boot a RHEL-4 guest on latest KVM tip on an AMD machine, I
found that the guest would consistently crash when trying to setup the NMI
watchdog. I traced it down to the following commit:

51ef1ac7b23ee32bfcc61c229d634fdc1c68b38a

It seems that in that commit, the K7_EVNTSEL MSR's were set to fail if the data
!= 0. That test is actually fine, the problem is how the code around it is
generated. That is, we are only supposed to go to unhandled if data != 0; but
for some reason, we are *always* going to unhandled, even when the data == 0.
That causes RHEL-4 kernel to crash. If I rearrange the code to look like this:

case MSR_K7_EVNTSEL0:
case MSR_K7_EVNTSEL1:
case MSR_K7_EVNTSEL2:
case MSR_K7_EVNTSEL3:

if (data != 0)
return kvm_set_msr_common(vcpu, ecx, data);

default:
return kvm_set_msr_common(vcpu, ecx, data);
}

Then everything works again. A patch that does just this is attached. It might
be slightly nicer to say "if (data == 0) return 0" and then just fall through to
the default case, but I don't much care either way.

Signed-off-by: Chris Lalancette <clalance@redhat.com>

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diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 89e0be2..ef550d7 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1291,10 +1291,9 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, unsigned ecx, u64 data)
* performance counter emulation later.
*/
if (data != 0)
- goto unhandled;
- break;
+ return kvm_set_msr_common(vcpu, ecx, data);
+
default:
- unhandled:
return kvm_set_msr_common(vcpu, ecx, data);
}
return 0;

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