[PATCH] KVM: x86: Use adjustment in guest cycles when handling MSR_IA32_TSC_ADJUST

[PATCH] KVM: x86: Use adjustment in guest cycles when handling MSR_IA32_TSC_ADJUST

[Haozhong Zhang]

When kvm_set_msr_common() handles a guest's write to
MSR_IA32_TSC_ADJUST, it will calcuate an adjustment based on the data
written by guest and then use it to adjust TSC offset by calling a
call-back adjust_tsc_offset(). The 3rd parameter of adjust_tsc_offset()
indicates whether the adjustment is in host TSC cycles or in guest TSC
cycles. If SVM TSC scaling is enabled, adjust_tsc_offset()
[i.e. svm_adjust_tsc_offset()] will first scale the adjustment;
otherwise, it will just use the unscaled one. As the MSR write here
comes from the guest, the adjustment is in guest TSC cycles. However,
the current kvm_set_msr_common() uses it as a value in host TSC
cycles (by using true as the 3rd parameter of adjust_tsc_offset()),
which can result in an incorrect adjustment of TSC offset if SVM TSC
scaling is enabled. This patch fixes this problem.

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
---
 arch/x86/kvm/x86.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5ef2560..3cd6cd2 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2105,7 +2105,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		if (guest_cpuid_has_tsc_adjust(vcpu)) {
 			if (!msr_info->host_initiated) {
 				s64 adj = data - vcpu->arch.ia32_tsc_adjust_msr;
-				kvm_x86_ops->adjust_tsc_offset(vcpu, adj, true);
+				adjust_tsc_offset_guest(vcpu, adj);
 			}
 			vcpu->arch.ia32_tsc_adjust_msr = data;
 		}
--
1.7.9.5

Re: [PATCH] KVM: x86: Use adjustment in guest cycles when handling MSR_IA32_TSC_ADJUST

[Paolo Bonzini]

On 07/08/2015 05:24, Haozhong Zhang wrote:
> When kvm_set_msr_common() handles a guest's write to
> MSR_IA32_TSC_ADJUST, it will calcuate an adjustment based on the data
> written by guest and then use it to adjust TSC offset by calling a
> call-back adjust_tsc_offset(). The 3rd parameter of adjust_tsc_offset()
> indicates whether the adjustment is in host TSC cycles or in guest TSC
> cycles. If SVM TSC scaling is enabled, adjust_tsc_offset()
> [i.e. svm_adjust_tsc_offset()] will first scale the adjustment;
> otherwise, it will just use the unscaled one. As the MSR write here
> comes from the guest, the adjustment is in guest TSC cycles. However,
> the current kvm_set_msr_common() uses it as a value in host TSC
> cycles (by using true as the 3rd parameter of adjust_tsc_offset()),
> which can result in an incorrect adjustment of TSC offset if SVM TSC
> scaling is enabled. This patch fixes this problem.
> 
> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> ---
>  arch/x86/kvm/x86.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5ef2560..3cd6cd2 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2105,7 +2105,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		if (guest_cpuid_has_tsc_adjust(vcpu)) {
>  			if (!msr_info->host_initiated) {
>  				s64 adj = data - vcpu->arch.ia32_tsc_adjust_msr;
> -				kvm_x86_ops->adjust_tsc_offset(vcpu, adj, true);
> +				adjust_tsc_offset_guest(vcpu, adj);
>  			}
>  			vcpu->arch.ia32_tsc_adjust_msr = data;
>  		}
> --
> 1.7.9.5
> 
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Thanks, applied!

Paolo