KVM: x86 emulator: Add new mode of instruction emulation: skip

In the new mode instruction is decoded, but not executed. The EIP
is moved to point after the instruction.
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -562,6 +562,7 @@ enum emulation_result {
 
 #define EMULTYPE_NO_DECODE	    (1 << 0)
 #define EMULTYPE_TRAP_UD	    (1 << 1)
+#define EMULTYPE_SKIP		    (1 << 2)
 int emulate_instruction(struct kvm_vcpu *vcpu, struct kvm_run *run,
 			unsigned long cr2, u16 error_code, int emulation_type);
 void kvm_report_emulation_failure(struct kvm_vcpu *cvpu, const char *context);

arch/x86/kvm/x86.c

@@ -2413,6 +2413,11 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 		}
 	}
 
+	if (emulation_type & EMULTYPE_SKIP) {
+		kvm_rip_write(vcpu, vcpu->arch.emulate_ctxt.decode.eip);
+		return EMULATE_DONE;
+	}
+
 	r = x86_emulate_insn(&vcpu->arch.emulate_ctxt, &emulate_ops);
 
 	if (vcpu->arch.pio.string)