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SubjectRe: [PATCH v4 2/6] KVM: x86: nSVM: implement nested LBR virtualization
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On 3/27/22 17:12, Maxim Levitsky wrote:
> - with LBR virtualization supported, the guest can set this msr to any value
> as long as it doesn't set reserved bits and then read back the written value,
> but it is not used by the CPU, unless LBR bit is set in MSR_IA32_DEBUGCTLMSR,
> because only then LBR virtualization is enabled, which makes the CPU
> load the guest value on VM entry.
>
> This means that MSR_IA32_DEBUGCTLMSR.BTF will magically start working when
> MSR_IA32_DEBUGCTLMSR.LBR is set as well, and will not work otherwise.

That can be fixed by context-switching DEBUGCTLMSR by hand when LBR=0 &&
BTF=1. Would you like to give it a shot?

Paolo

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