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>

diff --git a/drivers/kvm/kvm.h b/drivers/kvm/kvm.h
index f99e89e185b20c7ba20f3da857c932ab9717d5a4..41634fde8e13f0e594520bdde5f7be066e9cd57e 100644 (file)
--- a/drivers/kvm/kvm.h
+++ b/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;
 

diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
index cdf0b176851db8e1f1270a48011c4f8dc000b401..f267dbb52845f4b3b6c16119c62a34f3302ddfb8 100644 (file)
--- a/drivers/kvm/kvm_main.c
+++ b/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;