Re: [Qemu-devel] [PATCH 03/20] char: add IOWatchPoll support

[Anthony Liguori <anthony@codemonkey.ws>]

Amit Shah <amit.shah@redhat.com> writes:

> On (Fri) 29 Mar 2013 [07:24:07], Anthony Liguori wrote:
>> Amit Shah <amit.shah@redhat.com> writes:
>> 
>> > On (Tue) 05 Mar 2013 [23:21:18], Amit Shah wrote:
>> >> From: Anthony Liguori <aliguori@us.ibm.com>
>> >> 
>> >> This is a special GSource that supports CharDriverState style
>> >> poll callbacks.
>> >> 
>> >> For reviewability and bisectability, this code is #if 0'd out in this
>> >> patch to avoid unused warnings since all of the functions are static.
>> >> 
>> >> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
>> >> Signed-off-by: Amit Shah <amit.shah@redhat.com>
>> >
>> >
>> >> +static int io_channel_send_all(GIOChannel *fd, const void *_buf, int len1)
>> >> +{
>> >> +    GIOStatus status;
>> >> +    gsize bytes_written;
>> >> +    int len;
>> >> +    const uint8_t *buf = _buf;
>> >> +
>> >> +    len = len1;
>> >> +    while (len > 0) {
>> >> +        status = g_io_channel_write_chars(fd, (const gchar *)buf, len,
>> >> +                                          &bytes_written, NULL);
>> >> +        if (status != G_IO_STATUS_NORMAL) {
>> >> +            if (status != G_IO_STATUS_AGAIN) {
>> >> +                return -1;
>> >> +            }
>> >
>> > It's not quite right to return -1 here; previous iterations of the
>> > while loop could have successfully written data, and (len1 - len)
>> > could be +ve.
>> 
>> Once -1 is returned, it's a terminal error.  It doesn't matter that we
>> may have written some data.
>
> Why do you say that?

Because you're quoting the wrong patch :-)  This bit is rewritten by a
later patch which is the source of your problem below.  In the patch you
quote, we busy spin until all data is written.  However, with:

    commit 23673ca740e0eda66901ca801a5a901df378b063
    Author: Anthony Liguori <aliguori@us.ibm.com>
    Date:   Tue Mar 5 23:21:23 2013 +0530
    
        qemu-char: add watch support

We started to return EAGAIN even if we have a partially successful
write.  I'm running a patch through testing right now that rewrites this
function to have sane semantics (return bytes written on partial write).

I'll post as soon as testing completes.

Regards,

Anthony Liguori