KVM: VMX: Reset DR6 only when KVM_DEBUGREG_WONT_EXIT

The commit efdab992 ("KVM: x86: fix escape of guest dr6 to the host")
fixed a bug by resetting DR6 unconditionally when the vcpu being scheduled out.

But writing to debug registers is slow, and it can be visible in perf results
sometimes, even if neither the host nor the guest activate breakpoints.

Since KVM_DEBUGREG_WONT_EXIT on Intel processors is the only case
where DR6 gets the guest value, and it never happens at all on SVM,
the register can be cleared in vmx.c right after reading it.
Reported-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -5110,6 +5110,12 @@ static void vmx_sync_dirty_debug_regs(struct kvm_vcpu *vcpu)
 
 	vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_WONT_EXIT;
 	exec_controls_setbit(to_vmx(vcpu), CPU_BASED_MOV_DR_EXITING);
+
+	/*
+	 * exc_debug expects dr6 to be cleared after it runs, avoid that it sees
+	 * a stale dr6 from the guest.
+	 */
+	set_debugreg(DR6_RESERVED, 6);
 }
 
 static void vmx_set_dr7(struct kvm_vcpu *vcpu, unsigned long val)

arch/x86/kvm/x86.c

@@ -4310,12 +4310,6 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 
 	static_call(kvm_x86_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.
-	 */
-	set_debugreg(0, 6);
 }
 
 static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu,