linux-kernel - Re: [PATCH] KVM: SVM: fix trashing of MSR_TSC

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Date:	Fri, 15 Jul 2016 14:15:28 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	stable@...r.kernel.org, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] KVM: SVM: fix trashing of MSR_TSC_AUX

2016-07-06 15:43+0200, Paolo Bonzini:
> I don't know what I was thinking when I wrote commit 46896c73c1a4 ("KVM:
> svm: add support for RDTSCP", 2015-11-12); I missed write_rdtscp_aux which
> obviously uses MSR_TSC_AUX.
> 
> Therefore we do need to save/restore MSR_TSC_AUX in svm_vcpu_run.

Hm, MSR_TSC_AUX is in host_save_user_msrs[], so we save it on every
svm_vcpu_load() and restore on svm_vcpu_put().  Linux does not use
RDTSCP and every transition to userspace has svm_vcpu_put() in between.

We also still do "wrmsrl(MSR_TSC_AUX, svm->tsc_aux);" in svm_set_msr()
and can switch to userspace without performing svm_vcpu_run() first.

Was this patch fixing the host userspace or something in the guest?

Thanks.

> Cc: stable@...r.kernel.org
> Cc: Borislav Petkov <bp@...en8.de>
> Fixes: 46896c73c1a4 ("KVM: svm: add support for RDTSCP")
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/svm.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 16ef31b87452..44f6368f8b45 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -43,6 +43,7 @@
>  #include <asm/kvm_para.h>
>  
>  #include <asm/virtext.h>
> +#include <asm/vgtod.h>
>  #include "trace.h"
>  
>  #define __ex(x) __kvm_handle_fault_on_reboot(x)
> @@ -1530,9 +1531,6 @@ 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);
>  
>  	avic_vcpu_load(vcpu, cpu);
>  }
> @@ -4474,6 +4472,8 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>  	svm->vmcb->save.cr2 = vcpu->arch.cr2;
>  
>  	clgi();
> +	if (static_cpu_has(X86_FEATURE_RDTSCP))
> +		wrmsrl(MSR_TSC_AUX, svm->tsc_aux);
>  
>  	local_irq_enable();
>  
> @@ -4550,6 +4550,8 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>  #endif
>  		);
>  
> +	if (static_cpu_has(X86_FEATURE_RDTSCP))
> +		wrmsrl(MSR_TSC_AUX, __getcpu());
>  #ifdef CONFIG_X86_64
>  	wrmsrl(MSR_GS_BASE, svm->host.gs_base);
>  #else
> -- 
> 1.8.3.1
> 
> --
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