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

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Message-ID: <af5f7672-2934-a8f1-f3ed-51dce66aa5b4@redhat.com>
Date:	Fri, 15 Jul 2016 14:30:46 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...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



On 15/07/2016 14:15, Radim Krčmář wrote:
> 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?

I think I was helping someone to debug an AMD failure and missed the
addition to host_save_user_msrs in commit 46896c73c1a4.  So feel free to
revert, I guess.  Sorry for the mess.

Paolo

> 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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