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

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Message-ID: <095fe2d0-5ce4-4e0f-8f1b-6f7d14a20342@amd.com>
Date: Thu, 27 Feb 2025 19:29:43 +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

Hi Sean,

> @@ -4265,6 +4265,16 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu,
>  	clgi();
>  	kvm_load_guest_xsave_state(vcpu);
>  
> +	/*
> +	 * Hardware only context switches DEBUGCTL if LBR virtualization is
> +	 * enabled.  Manually load DEBUGCTL if necessary (and restore it after
> +	 * VM-Exit), as running with the host's DEBUGCTL can negatively affect
> +	 * guest state and can even be fatal, e.g. due to Bus Lock Detect.
> +	 */
> +	if (!(svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK) &&
> +	    vcpu->arch.host_debugctl != svm->vmcb->save.dbgctl)
> +		update_debugctlmsr(0);

                ^^^^^^^^^^^^^^^^^^^^^
You mean:
                update_debugctlmsr(svm->vmcb->save.dbgctl);
?

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;
+
 		kvm_queue_exception_p(vcpu, DB_VECTOR, payload);
 		return 1;
 	}
---

Thanks,
Ravi