[PATCH V2 3/3] KVM: X86: Reset DR6 only when KVM_DEBUGREG_WONT_EXIT

[Lai Jiangshan]

From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

The commit efdab992813fb ("KVM: x86: fix escape of guest dr6 to the host")
fixed a bug by reseting DR6 unconditionally when the vcpu being scheduled out.

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

It'd be better to reset it conditionally and this patch moves the code of
reseting DR6 to the path of VM-exit and only reset it when
KVM_DEBUGREG_WONT_EXIT which is the only case that DR6 is guest value.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d2aa49722064..f40cdd7687d8 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4309,12 +4309,6 @@ 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,
@@ -9630,6 +9624,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		static_call(kvm_x86_sync_dirty_debug_regs)(vcpu);
 		kvm_update_dr0123(vcpu);
 		kvm_update_dr7(vcpu);
+		/* Reset Dr6 which is guest value. */
+		set_debugreg(DR6_RESERVED, 6);
 	}
 
 	/*
-- 
2.19.1.6.gb485710b