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

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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>
This commit is contained in:
Paolo Bonzini 2014-02-21 10:32:27 +01:00
parent 5315c716b6
commit facb013969
1 changed files with 28 additions and 0 deletions
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28
arch/x86/kvm/svm.c
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@ -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,