[PATCH 4/4] KVM: vmx: Allow the guest to run with dirty debug registers

[Paolo Bonzini <pbonzini@redhat.com>]

When not running in guest-debug mode (i.e. the guest controls the debug
registers, having to take an exit for each DR access is a waste of time.
If the guest gets into a state where each context switch causes DR to be
saved and restored, this can take away as much as 40% of the execution
time from the guest.

If the guest is running with vcpu->arch.db == vcpu->arch.eff_db, we
can let it write freely to the debug registers and reload them on the
next exit.  We still need to exit on the first access, so that the
KVM_DEBUGREG_WONT_EXIT flag is set in switch_db_regs; after that, further
accesses to the debug registers will not cause a vmexit.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/kvm/vmx.c | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 6e57e1434cf3..71c57ec48d8f 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2851,7 +2851,7 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf)
 		      vmx_capability.ept, vmx_capability.vpid);
 	}
 
-	min = 0;
+	min = VM_EXIT_SAVE_DEBUG_CONTROLS;
 #ifdef CONFIG_X86_64
 	min |= VM_EXIT_HOST_ADDR_SPACE_SIZE;
 #endif
@@ -5121,6 +5121,22 @@ static int handle_dr(struct kvm_vcpu *vcpu)
 		}
 	}
 
+	if (vcpu->guest_debug == 0) {
+		u32 cpu_based_vm_exec_control;
+
+		cpu_based_vm_exec_control = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
+		cpu_based_vm_exec_control &= ~CPU_BASED_MOV_DR_EXITING;
+		vmcs_write32(CPU_BASED_VM_EXEC_CONTROL, cpu_based_vm_exec_control);
+
+		/*
+		 * 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.
+		 */
+		vcpu->arch.switch_db_regs |= KVM_DEBUGREG_WONT_EXIT;
+		return 1;
+	}
+
 	exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
 	dr = exit_qualification & DEBUG_REG_ACCESS_NUM;
 	reg = DEBUG_REG_ACCESS_REG(exit_qualification);
@@ -5147,6 +5163,18 @@ static void vmx_set_dr6(struct kvm_vcpu *vcpu, unsigned long val)
 {
 }
 
+static u64 vmx_get_dr7(struct kvm_vcpu *vcpu)
+{
+	/* DRs are being synced back to vcpu->arch, exit on DR access.  */
+	u32 cpu_based_vm_exec_control;
+
+	cpu_based_vm_exec_control = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
+	cpu_based_vm_exec_control |= CPU_BASED_MOV_DR_EXITING;
+	vmcs_write32(CPU_BASED_VM_EXEC_CONTROL, cpu_based_vm_exec_control);
+
+	return vmcs_readl(GUEST_DR7);
+}
+
 static void vmx_set_dr7(struct kvm_vcpu *vcpu, unsigned long val)
 {
 	vmcs_writel(GUEST_DR7, val);
@@ -8606,6 +8634,7 @@ static struct kvm_x86_ops vmx_x86_ops = {
 	.set_gdt = vmx_set_gdt,
 	.get_dr6 = vmx_get_dr6,
 	.set_dr6 = vmx_set_dr6,
+	.get_dr7 = vmx_get_dr7,
 	.set_dr7 = vmx_set_dr7,
 	.cache_reg = vmx_cache_reg,
 	.get_rflags = vmx_get_rflags,
-- 
1.8.3.1