KVM: emulate: speed up do_insn_fetch

Hoist the common case up from do_insn_fetch_byte to do_insn_fetch,
and prime the fetch_cache in x86_decode_insn.  This helps a bit the
compiler and the branch predictor, but above all it lays the
ground for further changes in the next few patches.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -705,25 +705,23 @@ static int segmented_read_std(struct x86_emulate_ctxt *ctxt,
 }
 
 /*
- * Fetch the next byte of the instruction being emulated which is pointed to
- * by ctxt->_eip, then increment ctxt->_eip.
- *
- * Also prefetch the remaining bytes of the instruction without crossing page
+ * Prefetch the remaining bytes of the instruction without crossing page
  * boundary if they are not in fetch_cache yet.
  */
-static int do_insn_fetch_byte(struct x86_emulate_ctxt *ctxt, u8 *dest)
+static int do_insn_fetch_bytes(struct x86_emulate_ctxt *ctxt)
 {
 	struct fetch_cache *fc = &ctxt->fetch;
 	int rc;
 	int size, cur_size;
-
-	if (ctxt->_eip == fc->end) {
 	unsigned long linear;
+
 	struct segmented_address addr = { .seg = VCPU_SREG_CS,
-						  .ea  = ctxt->_eip };
+					  .ea  = fc->end };
 	cur_size = fc->end - fc->start;
 	size = min(15UL - cur_size,
-			   PAGE_SIZE - offset_in_page(ctxt->_eip));
+		   PAGE_SIZE - offset_in_page(fc->end));
+	if (unlikely(size == 0))
+		return X86EMUL_UNHANDLEABLE;
 	rc = __linearize(ctxt, addr, size, false, true, &linear);
 	if (unlikely(rc != X86EMUL_CONTINUE))
 		return rc;
@@ -732,25 +730,27 @@ static int do_insn_fetch_byte(struct x86_emulate_ctxt *ctxt, u8 *dest)
 	if (unlikely(rc != X86EMUL_CONTINUE))
 		return rc;
 	fc->end += size;
-	}
-	*dest = fc->data[ctxt->_eip - fc->start];
-	ctxt->_eip++;
 	return X86EMUL_CONTINUE;
 }
 
 static int do_insn_fetch(struct x86_emulate_ctxt *ctxt,
-			 void *dest, unsigned size)
+			 void *__dest, unsigned size)
 {
 	int rc;
+	struct fetch_cache *fc = &ctxt->fetch;
+	u8 *dest = __dest;
+	u8 *src = &fc->data[ctxt->_eip - fc->start];
 
-	/* x86 instructions are limited to 15 bytes. */
-	if (unlikely(ctxt->_eip + size - ctxt->eip > 15))
-		return X86EMUL_UNHANDLEABLE;
 	while (size--) {
-		rc = do_insn_fetch_byte(ctxt, dest++);
+		if (unlikely(ctxt->_eip == fc->end)) {
+			rc = do_insn_fetch_bytes(ctxt);
 			if (rc != X86EMUL_CONTINUE)
 				return rc;
 		}
+		*dest++ = *src++;
+		ctxt->_eip++;
+		continue;
+	}
 	return X86EMUL_CONTINUE;
 }
 
@@ -4227,6 +4227,11 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
 	ctxt->opcode_len = 1;
 	if (insn_len > 0)
 		memcpy(ctxt->fetch.data, insn, insn_len);
+	else {
+		rc = do_insn_fetch_bytes(ctxt);
+		if (rc != X86EMUL_CONTINUE)
+			return rc;
+	}
 
 	switch (mode) {
 	case X86EMUL_MODE_REAL: