KVM: emulate: speed up emulated moves

We can just blindly move all 16 bytes of ctxt->src's value to ctxt->dst.
write_register_operand will take care of writing only the lower bytes.

Avoiding a call to memcpy (the compiler optimizes it out) gains about
200 cycles on kvm-unit-tests for register-to-register moves, and makes
them about as fast as arithmetic instructions.

We could perhaps get a larger speedup by moving all instructions _except_
moves out of x86_emulate_insn, removing opcode_len, and replacing the
switch statement with an inlined em_mov.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_emulate.h

@@ -233,7 +233,7 @@ struct operand {
 	union {
 		unsigned long val;
 		u64 val64;
-		char valptr[sizeof(unsigned long) + 2];
+		char valptr[sizeof(sse128_t)];
 		sse128_t vec_val;
 		u64 mm_val;
 		void *data;

arch/x86/kvm/emulate.c

@@ -2990,7 +2990,7 @@ static int em_rdpmc(struct x86_emulate_ctxt *ctxt)
 
 static int em_mov(struct x86_emulate_ctxt *ctxt)
 {
-	memcpy(ctxt->dst.valptr, ctxt->src.valptr, ctxt->op_bytes);
+	memcpy(ctxt->dst.valptr, ctxt->src.valptr, sizeof(ctxt->src.valptr));
 	return X86EMUL_CONTINUE;
 }