[PATCH 7/7] KVM: svm: Allow the guest to run with dirty debug registers

Paolo Bonzini
2014-03-07T11:43:17Z

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/svm.c | 27 +++++++++++++++++++++++++++
1 file changed, 27 insertions(+)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 5ed67e9739f6..ac68edccc57c 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1683,6 +1683,21 @@ static void svm_set_dr6(struct kvm_vcpu *vcpu, unsigned long value)
mark_dirty(svm->vmcb, VMCB_DR);
}

+static void svm_sync_dirty_debug_regs(struct kvm_vcpu *vcpu)
+{
+ struct vcpu_svm *svm = to_svm(vcpu);
+
+ get_debugreg(vcpu->arch.db[0], 0);
+ get_debugreg(vcpu->arch.db[1], 1);
+ get_debugreg(vcpu->arch.db[2], 2);
+ get_debugreg(vcpu->arch.db[3], 3);
+ vcpu->arch.dr6 = svm_get_dr6(vcpu);
+ vcpu->arch.dr7 = svm->vmcb->save.dr7;
+
+ vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_WONT_EXIT;
+ set_dr_intercepts(svm);
+}
+
static void svm_set_dr7(struct kvm_vcpu *vcpu, unsigned long value)
{
struct vcpu_svm *svm = to_svm(vcpu);
@@ -2974,6 +2989,17 @@ static int dr_interception(struct vcpu_svm *svm)
unsigned long val;
int err;

+ 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.
+ */
+ clr_dr_intercepts(svm);
+ svm->vcpu.arch.switch_db_regs |= KVM_DEBUGREG_WONT_EXIT;
+ return 1;
+ }
+
if (!boot_cpu_has(X86_FEATURE_DECODEASSISTS))
return emulate_on_interception(svm);

@@ -4302,6 +4328,7 @@ static struct kvm_x86_ops svm_x86_ops = {
.get_dr6 = svm_get_dr6,
.set_dr6 = svm_set_dr6,
.set_dr7 = svm_set_dr7,
+ .sync_dirty_debug_regs = svm_sync_dirty_debug_regs,
.cache_reg = svm_cache_reg,
.get_rflags = svm_get_rflags,
.set_rflags = svm_set_rflags,
--
1.8.3.1