KVM: x86 emulator: check io permissions only once for string pio

Do not recheck io permission on every iteration.
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/include/asm/kvm_emulate.h

@@ -224,6 +224,7 @@ struct x86_emulate_ctxt {
 	int interruptibility;
 
 	bool restart; /* restart string instruction after writeback */
+	bool perm_ok; /* do not check permissions if true */
 
 	int exception; /* exception that happens during emulation or -1 */
 	u32 error_code; /* error code for exception */

arch/x86/kvm/emulate.c

@@ -1621,9 +1621,15 @@ static bool emulator_io_permited(struct x86_emulate_ctxt *ctxt,
 				 struct x86_emulate_ops *ops,
 				 u16 port, u16 len)
 {
+	if (ctxt->perm_ok)
+		return true;
+
 	if (emulator_bad_iopl(ctxt, ops))
 		if (!emulator_io_port_access_allowed(ctxt, ops, port, len))
 			return false;
+
+	ctxt->perm_ok = true;
+
 	return true;
 }
 

arch/x86/kvm/x86.c

@@ -3997,6 +3997,7 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 		memcpy(c->regs, vcpu->arch.regs, sizeof c->regs);
 		vcpu->arch.emulate_ctxt.interruptibility = 0;
 		vcpu->arch.emulate_ctxt.exception = -1;
+		vcpu->arch.emulate_ctxt.perm_ok = false;
 
 		r = x86_decode_insn(&vcpu->arch.emulate_ctxt);
 		trace_kvm_emulate_insn_start(vcpu);