[Qemu-devel] [PATCH 3/4] Improve character device read performance

[Anthony Liguori <anthony@codemonkey.ws>]

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Regards,

Anthony Liguori

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# HG changeset patch
# User anthony@localhost.localdomain
# Node ID 32d5f77cf5e8c5a15b38b1fa86f588f374ebd3ad
# Parent  0e52f303222037247747558d43d72b5b89817828
Sometimes partial reads occur even though there's still data to be read.  This
patch makes sure that we read all the data that's available to read.

This has a huge performance impact for the shmem character device since.

diff -r 0e52f3032220 -r 32d5f77cf5e8 vl.c
--- a/vl.c	Sun Jul 16 17:04:01 2006
+++ b/vl.c	Sun Jul 16 17:23:21 2006
@@ -1205,6 +1205,29 @@
     return s->max_size;
 }
 
+static int fd_read_blocks(int fd)
+{
+    fd_set rfds;
+    struct timeval tv;
+    int ret;
+
+ again:
+    FD_ZERO(&rfds);
+    FD_SET(fd, &rfds);
+
+    tv.tv_sec = 0;
+    tv.tv_usec = 0;
+
+    ret = select(fd + 1, &rfds, NULL, NULL, &tv);
+    if (ret == -1) {
+	if (errno == EINTR)
+	    goto again;
+	return -1;
+    }
+
+    return !FD_ISSET(fd, &rfds);
+}
+
 static void fd_chr_read(void *opaque)
 {
     CharDriverState *chr = opaque;
@@ -1217,7 +1240,25 @@
         len = s->max_size;
     if (len == 0)
         return;
-    size = read(s->fd_in, buf, len);
+
+    for (size = 0; size < len;) {
+	int l;
+	l = read(s->fd_in, buf + size, len - size);
+	if (l == -1) {
+	    if (errno == EINTR)
+		continue;
+	    break;
+	} else if (l == 0) {
+	    break;
+	}
+
+	size += l;
+
+	if (size < len && fd_read_blocks(s->fd_in))
+	    break;
+    }
+
+
     if (size > 0) {
         s->fd_read(s->fd_opaque, buf, size);
     }