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Re: [Qemu-devel] [PATCH 2/2] main: switch qemu_set_fd_handler to g_io_ad


From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH 2/2] main: switch qemu_set_fd_handler to g_io_add_watch
Date: Sun, 04 Sep 2011 10:01:40 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.17) Gecko/20110516 Lightning/1.0b2 Thunderbird/3.1.10

On 09/04/2011 09:03 AM, Avi Kivity wrote:
On 08/22/2011 04:12 PM, Anthony Liguori wrote:
This patch changes qemu_set_fd_handler to be implemented in terms of
g_io_add_watch(). The semantics are a bit different so some glue is
required.

qemu_set_fd_handler2 is much harder to convert because of its use of
polling.

The glib main loop has the major of advantage of having a proven
thread safe
architecture. By using the glib main loop instead of our own, it will
allow us
to eventually introduce multiple I/O threads.

I'm pretty sure that this will work on Win32, but I would appreciate
some help
testing. I think the semantics of g_io_channel_unix_new() are really
just tied
to the notion of a "unix fd" and not necessarily unix itself.

'git bisect' fingered this as responsible for breaking
qcow2+cache=unsafe. I think there's an off-by-one here and the guilty
patch is the one that switches the main loop, but that's just a guess.

The symptoms are that a guest that is restarted (new qemu process) after
install doesn't make it through grub - some image data didn't make it do
disk. With qcow2 and cache=unsafe that can easily happen through exit
notifiers not being run and the entire qcow2 metadata being thrown out
the window. Running with raw+cache=unsafe works.

Can you share your full command line?

Nothing that would be in the obvious path actually uses qemu_set_fd_handler...

Regards,

Anthony Liguori




diff --git a/iohandler.c b/iohandler.c
index 4deae1e..5ef66fb 100644
--- a/iohandler.c
+++ b/iohandler.c
@@ -80,12 +80,67 @@ int qemu_set_fd_handler2(int fd,
return 0;
}

+typedef struct IOTrampoline
+{
+ GIOChannel *chan;
+ IOHandler *fd_read;
+ IOHandler *fd_write;
+ void *opaque;
+ guint tag;
+} IOTrampoline;
+
+static gboolean fd_trampoline(GIOChannel *chan, GIOCondition cond,
gpointer opaque)
+{
+ IOTrampoline *tramp = opaque;
+
+ if (tramp->opaque == NULL) {
+ return FALSE;
+ }
+
+ if ((cond& G_IO_IN)&& tramp->fd_read) {
+ tramp->fd_read(tramp->opaque);
+ }
+
+ if ((cond& G_IO_OUT)&& tramp->fd_write) {
+ tramp->fd_write(tramp->opaque);
+ }
+
+ return TRUE;
+}
+
int qemu_set_fd_handler(int fd,
IOHandler *fd_read,
IOHandler *fd_write,
void *opaque)
{
- return qemu_set_fd_handler2(fd, NULL, fd_read, fd_write, opaque);
+ static IOTrampoline fd_trampolines[FD_SETSIZE];
+ IOTrampoline *tramp =&fd_trampolines[fd];
+
+ if (tramp->tag != 0) {
+ g_io_channel_unref(tramp->chan);
+ g_source_remove(tramp->tag);
+ }
+
+ if (opaque) {
+ GIOCondition cond = 0;
+
+ tramp->fd_read = fd_read;
+ tramp->fd_write = fd_write;
+ tramp->opaque = opaque;
+
+ if (fd_read) {
+ cond |= G_IO_IN | G_IO_ERR;
+ }
+
+ if (fd_write) {
+ cond |= G_IO_OUT | G_IO_ERR;
+ }
+
+ tramp->chan = g_io_channel_unix_new(fd);
+ tramp->tag = g_io_add_watch(tramp->chan, cond, fd_trampoline, tramp);
+ }
+
+ return 0;
}

void qemu_iohandler_fill(int *pnfds, fd_set *readfds, fd_set
*writefds, fd_set *xfds)






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