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Commit facb0139 authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

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: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 5315c716

arch/x86/kvm/svm.c

+28 −0
Original line number Diff line number Diff line
@@ -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,