[RFC PATCH 3/3] kvm: x86: skip DRn reload if previous VM exit is DR access VM exit

[Xiaoyao Li]

When DR access vm exit, there is no DRn change throughout VM exit to
next VM enter. Skip the DRn reload in this case and fix the comments.

Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
---
 arch/x86/kvm/svm/svm.c | 8 +++++---
 arch/x86/kvm/vmx/vmx.c | 8 +++++---
 arch/x86/kvm/x86.c     | 2 +-
 3 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 66123848448d..c6883a0bf8c3 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2287,9 +2287,11 @@ static int dr_interception(struct vcpu_svm *svm)
 
 	if (svm->vcpu.guest_debug == 0) {
 		/*
-		 * No more DR vmexits; force a reload of the debug registers
-		 * and reenter on this instruction.  The next vmexit will
-		 * retrieve the full state of the debug registers.
+		 * No more DR vmexits and reenter on this instruction.
+		 * The next vmexit will retrieve the full state of the debug
+		 * registers and re-enable DR vmexits.
+		 * No need to set KVM_DEBUGREG_NEED_RELOAD because no DRn change
+		 * since this DR vmexit.
 		 */
 		clr_dr_intercepts(svm);
 		svm->vcpu.arch.switch_db_regs |= KVM_DEBUGREG_WONT_EXIT;
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index aa1b8cf7c915..22eff8503048 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4967,9 +4967,11 @@ static int handle_dr(struct kvm_vcpu *vcpu)
 		exec_controls_clearbit(to_vmx(vcpu), CPU_BASED_MOV_DR_EXITING);
 
 		/*
-		 * No more DR vmexits; force a reload of the debug registers
-		 * and reenter on this instruction.  The next vmexit will
-		 * retrieve the full state of the debug registers.
+		 * No more DR vmexits and reenter on this instruction.
+		 * The next vmexit will retrieve the full state of the debug
+		 * registers and re-enable DR vmexits.
+		 * No need to set KVM_DEBUGREG_NEED_RELOAD because no DRn change
+		 * since this DR vmexit.
 		 */
 		vcpu->arch.switch_db_regs |= KVM_DEBUGREG_WONT_EXIT;
 		return 1;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 71264df64001..8983848cbf45 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8400,7 +8400,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	if (test_thread_flag(TIF_NEED_FPU_LOAD))
 		switch_fpu_return();
 
-	if (unlikely(vcpu->arch.switch_db_regs)) {
+	if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_NEED_RELOAD)) {
 		set_debugreg(0, 7);
 		set_debugreg(vcpu->arch.eff_db[0], 0);
 		set_debugreg(vcpu->arch.eff_db[1], 1);
-- 
2.20.1