linux-kernel - Re: [PATCH] KVM: x86: Use adjustment in guest cycles when handling MSR_IA32_TSC

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Message-ID: <55C49638.3060508@redhat.com>
Date:	Fri, 7 Aug 2015 13:27:52 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Haozhong Zhang <haozhong.zhang@...el.com>, kvm@...r.kernel.org
Cc:	Gleb Natapov <gleb@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Use adjustment in guest cycles when handling
 MSR_IA32_TSC_ADJUST



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@...el.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
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