linux-kernel - Re: [PATCH v2 3/5] KVM: SVM: Manually context switch DEBUGCTL if LBR virtualization is disabled

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Message-ID: <f560ecab-c1bd-483e-9cd6-f6fb4127b8b8@amd.com>
Date: Thu, 27 Feb 2025 20:14:21 +0530
From: Ravi Bangoria <ravi.bangoria@....com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, Xiaoyao Li <xiaoyao.li@...el.com>,
 rangemachine@...il.com, whanos@...gal.fun,
 Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [PATCH v2 3/5] KVM: SVM: Manually context switch DEBUGCTL if LBR
 virtualization is disabled

>> Somewhat related but independent: CPU automatically clears DEBUGCTL[BTF]
>> on #DB exception. So, when DEBUGCTL is save/restored by KVM (i.e. when
>> LBR virtualization is disabled), it's KVM's responsibility to clear
>> DEBUGCTL[BTF].
>> ---
>> @@ -2090,6 +2090,14 @@ static int db_interception(struct kvm_vcpu *vcpu)
>>  	      (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP)) &&
>>  		!svm->nmi_singlestep) {
>>  		u32 payload = svm->vmcb->save.dr6 ^ DR6_ACTIVE_LOW;
>> +
>> +		/*
>> +		 * CPU automatically clears DEBUGCTL[BTF] on #DB exception.
>> +		 * Simulate it when DEBUGCTL isn't auto save/restored.
>> +		 */
>> +		if (!(svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK))
>> +			svm->vmcb->save.dbgctl &= ~0x2;
> 
> Any reason not to clear is unconditionally?
> 
> 		svm->vmcb->save.dbgctl &= ~DEBUGCTLMSR_BTF;

No particular reason, just that HW would have already done it when LBRV
is enabled.

Thanks,
Ravi