Re: [Qemu-devel] [PATCH 2/2] open ptys in non-blocking mode.

[Gerd Hoffmann <kraxel@redhat.com>]

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Kevin Wolf wrote:
> [Crossposting to xen-devel]
> 
> Ian, we need something like this for qemu-xen (or ioemu-remote or
> whatever it's called now). Currently you must attach to the console of a
> domain, otherwise it won't boot up and keep hanging in a blocking write
> because the buffer is full.
> 
> The old ioemu had a hack in unix_write (doing a select before the write)
> which you didn't merge into qemu-xen. In fact, I noticed that you even
> removed that function entirely and I'm wondering why.

For completeness:  You also need the attached patch for unix_write,
otherwise you'll end up with qemu burning cpu cycles.  If you can't
write to a non-blocking file handle the write will instantly return with
-EAGAIN.  Calling it again of course doesn't change the result, so
better don't do that ...

-- 
http://kraxel.fedorapeople.org/xenner/

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>From d9454802fa2105bc399518eebfa1e1415ff07143 Mon Sep 17 00:00:00 2001
From: Gerd Hoffmann <kraxel@redhat.com>
Date: Wed, 9 Jul 2008 09:41:31 +0200
Subject: [PATCH 02/12] unix_write: don't block on non-blocking file handles.


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 vl.c |   12 +++++++++++-
 1 files changed, 11 insertions(+), 1 deletions(-)

diff --git a/vl.c b/vl.c
index c8db579..2e2d883 100644
--- a/vl.c
+++ b/vl.c
@@ -2116,14 +2116,24 @@ void socket_set_nonblock(int fd)
 
 static int unix_write(int fd, const uint8_t *buf, int len1)
 {
+    int nonblock = fcntl(fd, F_GETFL) & O_NONBLOCK;
     int ret, len;
 
     len = len1;
     while (len > 0) {
         ret = write(fd, buf, len);
         if (ret < 0) {
-            if (errno != EINTR && errno != EAGAIN)
+            if (errno == EINTR) {
+		continue;
+	    } else if (errno == EAGAIN) {
+		if (!nonblock)
+		    continue;
+		if (len1 != len)
+		    break; /* partial write, return written bytes */
+		return -1;
+	    } else {
                 return -1;
+	    }
         } else if (ret == 0) {
             break;
         } else {
-- 
1.5.4.1