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

[Avi Kivity]

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.


> 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)


--
error compiling committee.c: too many arguments to function