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SubjectRe: [PATCH] KVM: x86: nSVM: disallow userspace setting of MSR_AMD64_TSC_RATIO to non default value when tsc scaling disabled
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On 2/23/22 12:56, Maxim Levitsky wrote:
> If nested tsc scaling is disabled, MSR_AMD64_TSC_RATIO should
> never have non default value.
>
> Due to way nested tsc scaling support was implmented in qemu,
> it would set this msr to 0 when nested tsc scaling was disabled.
> Ignore that value for now, as it causes no harm.
>
>
> Fixes: 5228eb96a487 ("KVM: x86: nSVM: implement nested TSC scaling")
> Cc: stable@vger.kernel.org
>
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
> arch/x86/kvm/svm/svm.c | 19 +++++++++++++++++--
> 1 file changed, 17 insertions(+), 2 deletions(-)

Queued, thanks.

Paolo

> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 7038c76fa841..b80ad471776f 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2705,8 +2705,23 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
> u64 data = msr->data;
> switch (ecx) {
> case MSR_AMD64_TSC_RATIO:
> - if (!msr->host_initiated && !svm->tsc_scaling_enabled)
> - return 1;
> +
> + if (!svm->tsc_scaling_enabled) {
> +
> + if (!msr->host_initiated)
> + return 1;
> + /*
> + * In case TSC scaling is not enabled, always
> + * leave this MSR at the default value.
> + *
> + * Due to bug in qemu 6.2.0, it would try to set
> + * this msr to 0 if tsc scaling is not enabled.
> + * Ignore this value as well.
> + */
> + if (data != 0 && data != svm->tsc_ratio_msr)
> + return 1;
> + break;
> + }
>
> if (data & TSC_RATIO_RSVD)
> return 1;

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