KVM: x86 emulator: simplify read_emulated

No need split mmio read region into 8-bits pieces since we do it in
emulator_read_write_onepage

Changelog:
  Add a WARN_ON to check read-cache overflow
Acked-by: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NXiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>

arch/x86/kvm/emulate.c

@@ -1166,24 +1166,21 @@ static int read_emulated(struct x86_emulate_ctxt *ctxt,
 	int rc;
 	struct read_cache *mc = &ctxt->mem_read;
 
-	while (size) {
-		int n = min(size, 8u);
-		size -= n;
-		if (mc->pos < mc->end)
-			goto read_cached;
-
-		rc = ctxt->ops->read_emulated(ctxt, addr, mc->data + mc->end, n,
-					      &ctxt->exception);
-		if (rc != X86EMUL_CONTINUE)
-			return rc;
-		mc->end += n;
+	if (mc->pos < mc->end)
+		goto read_cached;
 
-	read_cached:
-		memcpy(dest, mc->data + mc->pos, n);
-		mc->pos += n;
-		dest += n;
-		addr += n;
-	}
+	WARN_ON((mc->end + size) >= sizeof(mc->data));
+
+	rc = ctxt->ops->read_emulated(ctxt, addr, mc->data + mc->end, size,
+				      &ctxt->exception);
+	if (rc != X86EMUL_CONTINUE)
+		return rc;
+
+	mc->end += size;
+
+read_cached:
+	memcpy(dest, mc->data + mc->pos, size);
+	mc->pos += size;
 	return X86EMUL_CONTINUE;
 }