KVM: emulator: do not needlesly sync registers from emulator ctxt to vcpu

Currently we sync registers back and forth before/after exiting
to userspace for IO, but during IO device model shouldn't need to
read/write the registers, so we can as well skip those sync points. The
only exaception is broken vmware backdor interface. The new code sync
registers content during IO only if registers are read from/written to
by userspace in the middle of the IO operation and this almost never
happens in practise.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/x86.c

@@ -4420,6 +4420,7 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
 	vcpu->arch.emulate_ctxt.guest_mode = is_guest_mode(vcpu);
 	memset(c, 0, sizeof(struct decode_cache));
 	memcpy(c->regs, vcpu->arch.regs, sizeof c->regs);
+	vcpu->arch.emulate_regs_need_sync_from_vcpu = false;
 }
 
 int kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq)
@@ -4502,6 +4503,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 {
 	int r;
 	struct decode_cache *c = &vcpu->arch.emulate_ctxt.decode;
+	bool writeback = true;
 
 	kvm_clear_exception_queue(vcpu);
 	vcpu->arch.mmio_fault_cr2 = cr2;
@@ -4542,9 +4544,12 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 		return EMULATE_DONE;
 	}
 
-	/* this is needed for vmware backdor interface to work since it
+	/* this is needed for vmware backdoor interface to work since it
 	   changes registers values  during IO operation */
-	memcpy(c->regs, vcpu->arch.regs, sizeof c->regs);
+	if (vcpu->arch.emulate_regs_need_sync_from_vcpu) {
+		vcpu->arch.emulate_regs_need_sync_from_vcpu = false;
+		memcpy(c->regs, vcpu->arch.regs, sizeof c->regs);
+	}
 
 restart:
 	r = x86_emulate_insn(&vcpu->arch.emulate_ctxt);
@@ -4565,19 +4570,28 @@ restart:
 	} else if (vcpu->arch.pio.count) {
 		if (!vcpu->arch.pio.in)
 			vcpu->arch.pio.count = 0;
+		else
+			writeback = false;
 		r = EMULATE_DO_MMIO;
-	} else if (vcpu->mmio_needed)
+	} else if (vcpu->mmio_needed) {
+		if (!vcpu->mmio_is_write)
+			writeback = false;
 		r = EMULATE_DO_MMIO;
-	else if (r == EMULATION_RESTART)
+	} else if (r == EMULATION_RESTART)
 		goto restart;
 	else
 		r = EMULATE_DONE;
 
-	toggle_interruptibility(vcpu, vcpu->arch.emulate_ctxt.interruptibility);
-	kvm_set_rflags(vcpu, vcpu->arch.emulate_ctxt.eflags);
-	kvm_make_request(KVM_REQ_EVENT, vcpu);
-	memcpy(vcpu->arch.regs, c->regs, sizeof c->regs);
-	kvm_rip_write(vcpu, vcpu->arch.emulate_ctxt.eip);
+	if (writeback) {
+		toggle_interruptibility(vcpu,
+				vcpu->arch.emulate_ctxt.interruptibility);
+		kvm_set_rflags(vcpu, vcpu->arch.emulate_ctxt.eflags);
+		kvm_make_request(KVM_REQ_EVENT, vcpu);
+		memcpy(vcpu->arch.regs, c->regs, sizeof c->regs);
+		vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
+		kvm_rip_write(vcpu, vcpu->arch.emulate_ctxt.eip);
+	} else
+		vcpu->arch.emulate_regs_need_sync_to_vcpu = true;
 
 	return r;
 }
@@ -5587,6 +5601,18 @@ out:
 
 int kvm_arch_vcpu_ioctl_get_regs(struct kvm_vcpu *vcpu, struct kvm_regs *regs)
 {
+	if (vcpu->arch.emulate_regs_need_sync_to_vcpu) {
+		/*
+		 * We are here if userspace calls get_regs() in the middle of
+		 * instruction emulation. Registers state needs to be copied
+		 * back from emulation context to vcpu. Usrapace shouldn't do
+		 * that usually, but some bad designed PV devices (vmware
+		 * backdoor interface) need this to work
+		 */
+		struct decode_cache *c = &vcpu->arch.emulate_ctxt.decode;
+		memcpy(vcpu->arch.regs, c->regs, sizeof c->regs);
+		vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
+	}
 	regs->rax = kvm_register_read(vcpu, VCPU_REGS_RAX);
 	regs->rbx = kvm_register_read(vcpu, VCPU_REGS_RBX);
 	regs->rcx = kvm_register_read(vcpu, VCPU_REGS_RCX);
@@ -5614,6 +5640,9 @@ int kvm_arch_vcpu_ioctl_get_regs(struct kvm_vcpu *vcpu, struct kvm_regs *regs)
 
 int kvm_arch_vcpu_ioctl_set_regs(struct kvm_vcpu *vcpu, struct kvm_regs *regs)
 {
+	vcpu->arch.emulate_regs_need_sync_from_vcpu = true;
+	vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
+
 	kvm_register_write(vcpu, VCPU_REGS_RAX, regs->rax);
 	kvm_register_write(vcpu, VCPU_REGS_RBX, regs->rbx);
 	kvm_register_write(vcpu, VCPU_REGS_RCX, regs->rcx);