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>
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1d5a7f4..3416a34 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4420,6 +4420,7 @@
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 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 @@
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 @@
} 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 @@
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_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);