linux-kernel - Re: [PATCH] KVM: x86: Trace changes to active TSC offset regardless if vCPU in guest-mode

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Date:   Mon, 26 Nov 2018 11:27:13 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH] KVM: x86: Trace changes to active TSC offset regardless
 if vCPU in guest-mode

On 25/11/18 22:38, Liran Alon wrote:
> 
> 
>> On 25 Nov 2018, at 19:53, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>> For some reason, kvm_x86_ops->write_l1_tsc_offset() skipped trace
>> of change to active TSC offset in case vCPU is in guest-mode.
>> This patch changes write_l1_tsc_offset() behavior to trace any change
>> to active TSC offset to aid debugging.  The VMX code is changed to
>> look more similar to SVM, which is in my opinion nicer.
>>
>> Based on a patch by Liran Alon.
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> 
> I would have applied this refactoring change on top of my original version of this patch. Easier to read and review.

Your original version of the patch is a bit different, in that it traced
the L2 TSC offset for vmx and the L1 TSC offset for svm.  I think
tracing the L1 TSC offset is clearer, but then doing it for vmx entailed
either rewriting the function twice, or squashing the refactoring in
this patch.

Paolo

> But I guess it’s a matter of taste…
> Anyway, code looks correct to me. Therefore:
> Reviewed-by: Liran Alon <liran.alon@...cle.com>
> 
>> ---
>> 	Untested still, but throwing it out because it seems pretty
>> 	obvious...
>>
>> arch/x86/kvm/svm.c |  9 +++++----
>> arch/x86/kvm/vmx.c | 34 +++++++++++++++++-----------------
>> 2 files changed, 22 insertions(+), 21 deletions(-)
>>
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> index a24733aade4c..0d1a74069a9e 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -1456,10 +1456,11 @@ static u64 svm_write_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
>> 		g_tsc_offset = svm->vmcb->control.tsc_offset -
>> 			       svm->nested.hsave->control.tsc_offset;
>> 		svm->nested.hsave->control.tsc_offset = offset;
>> -	} else
>> -		trace_kvm_write_tsc_offset(vcpu->vcpu_id,
>> -					   svm->vmcb->control.tsc_offset,
>> -					   offset);
>> +	}
>> +
>> +	trace_kvm_write_tsc_offset(vcpu->vcpu_id,
>> +				   svm->vmcb->control.tsc_offset - g_tsc_offset,
>> +				   offset);
>>
>> 	svm->vmcb->control.tsc_offset = offset + g_tsc_offset;
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 764c23dc444f..e7d3f7d35355 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -3466,24 +3466,24 @@ static u64 vmx_read_l1_tsc_offset(struct kvm_vcpu *vcpu)
>>
>> static u64 vmx_write_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
>> {
>> -	u64 active_offset = offset;
>> -	if (is_guest_mode(vcpu)) {
>> -		/*
>> -		 * We're here if L1 chose not to trap WRMSR to TSC. According
>> -		 * to the spec, this should set L1's TSC; The offset that L1
>> -		 * set for L2 remains unchanged, and still needs to be added
>> -		 * to the newly set TSC to get L2's TSC.
>> -		 */
>> -		struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>> -		if (nested_cpu_has(vmcs12, CPU_BASED_USE_TSC_OFFSETING))
>> -			active_offset += vmcs12->tsc_offset;
>> -	} else {
>> -		trace_kvm_write_tsc_offset(vcpu->vcpu_id,
>> -					   vmcs_read64(TSC_OFFSET), offset);
>> -	}
>> +	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>> +	u64 g_tsc_offset = 0;
>> +
>> +	/*
>> +	 * We're here if L1 chose not to trap WRMSR to TSC. According
>> +	 * to the spec, this should set L1's TSC; The offset that L1
>> +	 * set for L2 remains unchanged, and still needs to be added
>> +	 * to the newly set TSC to get L2's TSC.
>> +	 */
>> +	if (is_guest_mode(vcpu) &&
>> +	    (vmcs12->cpu_based_vm_exec_control & CPU_BASED_USE_TSC_OFFSETING))
>> +		g_tsc_offset = vmcs12->tsc_offset;
>>
>> -	vmcs_write64(TSC_OFFSET, active_offset);
>> -	return active_offset;
>> +	trace_kvm_write_tsc_offset(vcpu->vcpu_id,
>> +				   vcpu->arch.tsc_offset - g_tsc_offset,
>> +				   offset);
>> +	vmcs_write64(TSC_OFFSET, offset + g_tsc_offset);
>> +	return offset + g_tsc_offset;
>> }
>>
>> /*
>> -- 
>> 1.8.3.1
>>
> 
> 
>