KVM: Remove extraneous guest entry on mmio read

When emulating an mmio read, we actually emulate twice: once to determine
the physical address of the mmio, and, after we've exited to userspace to
get the mmio value, we emulate again to place the value in the result
register and update any flags.

But we don't really need to enter the guest again for that, only to take
an immediate vmexit.  So, if we detect that we're doing an mmio read,
emulate a single instruction before entering the guest again.
Signed-off-by: Avi Kivity <avi@qumranet.com>

drivers/kvm/kvm.h

@@ -310,6 +310,7 @@ struct kvm_vcpu {
 	int mmio_size;
 	unsigned char mmio_data[8];
 	gpa_t mmio_phys_addr;
+	gva_t mmio_fault_cr2;
 	struct kvm_pio_request pio;
 	void *pio_data;
 

drivers/kvm/kvm_main.c

@@ -1186,6 +1186,7 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 	int r;
 	int cs_db, cs_l;
 
+	vcpu->mmio_fault_cr2 = cr2;
 	kvm_arch_ops->cache_regs(vcpu);
 
 	kvm_arch_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
@@ -1804,14 +1805,23 @@ static int kvm_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 			r = complete_pio(vcpu);
 			if (r)
 				goto out;
-		} else {
+		} else if (!vcpu->mmio_is_write) {
 			memcpy(vcpu->mmio_data, kvm_run->mmio.data, 8);
 			vcpu->mmio_read_completed = 1;
+			vcpu->mmio_needed = 0;
+			r = emulate_instruction(vcpu, kvm_run,
+					    vcpu->mmio_fault_cr2, 0);
+			if (r == EMULATE_DO_MMIO) {
+				/*
+				 * Read-modify-write.  Back to userspace.
+				 */
+				kvm_run->exit_reason = KVM_EXIT_MMIO;
+				r = 0;
+				goto out;
+			}
 		}
 	}
 
-	vcpu->mmio_needed = 0;
-
 	if (kvm_run->exit_reason == KVM_EXIT_HYPERCALL) {
 		kvm_arch_ops->cache_regs(vcpu);
 		vcpu->regs[VCPU_REGS_RAX] = kvm_run->hypercall.ret;