kvm: x86: optimize dr6 restore (0e0a53c5) · Commits · 戴 / test

The quote from the comment almost says it all: we are currently zeroing the guest dr6 in kvm_arch_vcpu_put, because do_debug expects it. However, the host %dr6 is either: - zero because the guest hasn't run after kvm_arch_vcpu_load - written from vcpu->arch.dr6 by vcpu_enter_guest - written by the guest and copied to vcpu->arch.dr6 by ->sync_dirty_debug_regs(). Therefore, we can skip the write if vcpu->arch.dr6 is already zero. We may do extra useless writes if vcpu->arch.dr6 is nonzero but the guest hasn't run; however that is less important for performance. Signed-off-by:

Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

+9 −4

Original line number	Diff line number	Diff line
		@@ -3186,10 +3186,15 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
		kvm_x86_ops->vcpu_put(vcpu);
		vcpu->arch.last_host_tsc = rdtsc();
		/*
		* If userspace has set any breakpoints or watchpoints, dr6 is restored
		* on every vmexit, but if not, we might have a stale dr6 from the
		* guest. do_debug expects dr6 to be cleared after it runs, do the same.
		*/
		* Here dr6 is either zero or, if the guest has run and userspace
		* has not set any breakpoints or watchpoints, it can be set to
		* the guest dr6 (stored in vcpu->arch.dr6). do_debug expects dr6
		* to be cleared after it runs, so clear the host register. However,
		* MOV to DR can be expensive when running nested, omit it if
		* vcpu->arch.dr6 is already zero: in that case, the host dr6 cannot
		* currently be nonzero.
		*/
		if (vcpu->arch.dr6)
		set_debugreg(0, 6);
		}

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