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>

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index eec04129402f3a0eb7cb2064d3529d0e0a020676..f482100eff87bd44303306dc6fa53353a6595113 100644 (file)
--- 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)