linux-kernel - Re: [PATCH] KVM: svm: add support for RDTSCP

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Message-ID: <20151112144509.GD13027@8bytes.org>
Date:	Thu, 12 Nov 2015 15:45:09 +0100
From:	Joerg Roedel <joro@...tes.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: svm: add support for RDTSCP

Hi Paolo,

On Thu, Nov 12, 2015 at 02:49:16PM +0100, Paolo Bonzini wrote:
> RDTSCP was never supported for AMD CPUs, which nobody noticed because
> Linux does not use it.  But exactly the fact that Linux does not
> use it makes the implementation very simple; we can freely trash
> MSR_TSC_AUX while running the guest.
> 
> Cc: Joerg Roedel <joro@...tes.org>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/svm.c | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 83a1c64..c302614 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -86,6 +86,7 @@ static const u32 host_save_user_msrs[] = {
>  	MSR_FS_BASE,
>  #endif
>  	MSR_IA32_SYSENTER_CS, MSR_IA32_SYSENTER_ESP, MSR_IA32_SYSENTER_EIP,
> +	MSR_TSC_AUX,
>  };
>  
>  #define NR_HOST_SAVE_USER_MSRS ARRAY_SIZE(host_save_user_msrs)
> @@ -135,6 +136,7 @@ struct vcpu_svm {
>  	uint64_t asid_generation;
>  	uint64_t sysenter_esp;
>  	uint64_t sysenter_eip;
> +	uint64_t tsc_aux;
>  
>  	u64 next_rip;
>  
> @@ -1238,6 +1240,9 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  			wrmsrl(MSR_AMD64_TSC_RATIO, tsc_ratio);
>  		}
>  	}
> +	/* This assumes that the kernel never uses MSR_TSC_AUX */
> +	if (static_cpu_has(X86_FEATURE_RDTSCP))
> +		wrmsrl(MSR_TSC_AUX, svm->tsc_aux);
>  }

Hmm, you seem to still intercept MSR_TSC_AUX, is that intentional?
Loading the guests value into the real cpu msr only makes sense to me
when the MSR is no longer intercepted.


	Joerg

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