Re: [PATCH] KVM: x86: nSVM: disallow userspace setting of MSR_AMD64_TSC_RATIO to non default value when tsc scaling disabled

[Paolo Bonzini <pbonzini@redhat.com>]

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;