KVM: VMX: Conditionally reload debug register 6

Only reload debug register 6 if we're running with the guest's
debug registers.  Saves around 150 cycles from the guest lightweight
exit path.

dr6 contains a couple of bits that are updated on #DB, so intercept
that unconditionally and update those bits then.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 9 insertions, 5 deletions

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index eec04129402f..f482100eff87 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -540,10 +540,12 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu)
         eb = (1u << PF_VECTOR) | (1u << UD_VECTOR) | (1u << MC_VECTOR);
         if (!vcpu->fpu_active)
                 eb |= 1u << NM_VECTOR;
+        /*
+         * Unconditionally intercept #DB so we can maintain dr6 without
+         * reading it every exit.
+         */
+        eb |= 1u << DB_VECTOR;
         if (vcpu->guest_debug & KVM_GUESTDBG_ENABLE) {
-                if (vcpu->guest_debug &
-                    (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP))
-                        eb |= 1u << DB_VECTOR;
                 if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP)
                         eb |= 1u << BP_VECTOR;
         }
@@ -3632,7 +3634,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
          */
         vmcs_writel(HOST_CR0, read_cr0());
 
-        set_debugreg(vcpu->arch.dr6, 6);
+        if (vcpu->arch.switch_db_regs)
+                set_debugreg(vcpu->arch.dr6, 6);
 
         asm(
                 /* Store host registers */
@@ -3734,7 +3737,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
                                   | (1 << VCPU_EXREG_PDPTR));
         vcpu->arch.regs_dirty = 0;
 
-        get_debugreg(vcpu->arch.dr6, 6);
+        if (vcpu->arch.switch_db_regs)
+                get_debugreg(vcpu->arch.dr6, 6);
 
         vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
         if (vmx->rmode.irq.pending)