[PATCH 2/6] KVM: x86: Optimize debug register switching

[Jan Kiszka <jan.kiszka@xxxxxxxxxxx>]

Based on Avi's suggestion: Do not save the host debug registers on guest
entry as they are already present in the thread state. Moreover, only
restore them if the current host thread is being debugged. But as KGDB
accesses the debug register directly, we have to fall back to existing
pattern in that case.

Signed-off-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
---

 arch/x86/kvm/x86.c |   48 +++++++++++++++++++++++++++++++++---------------
 1 files changed, 33 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 891234b..036a2c5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -37,6 +37,7 @@
 #include <linux/iommu.h>
 #include <linux/intel-iommu.h>
 #include <linux/cpufreq.h>
+#include <linux/kgdb.h>
 #include <trace/events/kvm.h>
 #undef TRACE_INCLUDE_FILE
 #define CREATE_TRACE_POINTS
@@ -3627,14 +3628,21 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 
 	kvm_guest_enter();
 
-	get_debugreg(vcpu->arch.host_dr6, 6);
-	get_debugreg(vcpu->arch.host_dr7, 7);
+	/*
+	 * kgdb accesses the debug registers directly, so we have to save them
+	 * and restore those values on return from the guest.
+	 */
+	if (unlikely(kgdb_in_use())) {
+		if (unlikely(vcpu->arch.switch_db_regs)) {
+			get_debugreg(vcpu->arch.host_db[0], 0);
+			get_debugreg(vcpu->arch.host_db[1], 1);
+			get_debugreg(vcpu->arch.host_db[2], 2);
+			get_debugreg(vcpu->arch.host_db[3], 3);
+		}
+		get_debugreg(vcpu->arch.host_dr6, 6);
+		get_debugreg(vcpu->arch.host_dr7, 7);
+	}
 	if (unlikely(vcpu->arch.switch_db_regs)) {
-		get_debugreg(vcpu->arch.host_db[0], 0);
-		get_debugreg(vcpu->arch.host_db[1], 1);
-		get_debugreg(vcpu->arch.host_db[2], 2);
-		get_debugreg(vcpu->arch.host_db[3], 3);
-
 		set_debugreg(0, 7);
 		set_debugreg(vcpu->arch.eff_db[0], 0);
 		set_debugreg(vcpu->arch.eff_db[1], 1);
@@ -3645,15 +3653,25 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	trace_kvm_entry(vcpu->vcpu_id);
 	kvm_x86_ops->run(vcpu);
 
-	if (unlikely(vcpu->arch.switch_db_regs)) {
-		set_debugreg(0, 7);
-		set_debugreg(vcpu->arch.host_db[0], 0);
-		set_debugreg(vcpu->arch.host_db[1], 1);
-		set_debugreg(vcpu->arch.host_db[2], 2);
-		set_debugreg(vcpu->arch.host_db[3], 3);
+	if (unlikely(kgdb_in_use())) {
+		if (unlikely(vcpu->arch.switch_db_regs)) {
+			set_debugreg(vcpu->arch.host_db[0], 0);
+			set_debugreg(vcpu->arch.host_db[1], 1);
+			set_debugreg(vcpu->arch.host_db[2], 2);
+			set_debugreg(vcpu->arch.host_db[3], 3);
+		}
+		set_debugreg(vcpu->arch.host_dr6, 6);
+		set_debugreg(vcpu->arch.host_dr7, 7);
+	} else if (unlikely(test_thread_flag(TIF_DEBUG))) {
+		if (unlikely(vcpu->arch.switch_db_regs)) {
+			set_debugreg(current->thread.debugreg0, 0);
+			set_debugreg(current->thread.debugreg1, 1);
+			set_debugreg(current->thread.debugreg2, 2);
+			set_debugreg(current->thread.debugreg3, 3);
+		}
+		set_debugreg(current->thread.debugreg6, 6);
+		set_debugreg(current->thread.debugreg7, 7);
 	}
-	set_debugreg(vcpu->arch.host_dr6, 6);
-	set_debugreg(vcpu->arch.host_dr7, 7);
 
 	set_bit(KVM_REQ_KICK, &vcpu->requests);
 	local_irq_enable();

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