KVM: svm: Allow the guest to run with dirty debug registers

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>

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 86d802be602d33b583cb047462d2f64111ed0b0f..a449c3d76cba00890ee1fa83966f4bf14e258961 100644 (file)
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -34,6 +34,7 @@
 #include <asm/perf_event.h>
 #include <asm/tlbflush.h>
 #include <asm/desc.h>
+#include <asm/debugreg.h>
 #include <asm/kvm_para.h>
 
 #include <asm/virtext.h>
@@ -1683,6 +1684,21 @@ 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 +2990,17 @@ 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);
 
@@ -4300,6 +4327,7 @@ 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,