Re: [Qemu-devel] [PATCH v3 04/13] nbd: convert to using I/O channels for actual socket I/O

[Paolo Bonzini <pbonzini@redhat.com>]

On 19/01/2016 14:09, Daniel P. Berrange wrote:
> + * This will update @iov so that its head is advanced
> + * by @skip bytes. To do this, zero or more complete
> + * elements of @iov will be skipped over. The new head
> + * of @iov will then have its base & len updated to
> + * skip the remaining number of bytes. @oldiov will
> + * hold the original data from the new head of @iov.
> + */
> +static void nbd_skip_iov(size_t skip,
> +                         struct iovec **iov,
> +                         int *niov,
> +                         struct iovec *oldiov)
>  {
> -    size_t offset = 0;
> -    int err;
> -
> -    if (qemu_in_coroutine()) {
> -        if (do_read) {
> -            return qemu_co_recv(fd, buffer, size);
> +    ssize_t done = 0;
> +    size_t i;
> +    for (i = 0; i < *niov; i++) {
> +        if ((*iov)[i].iov_len <= skip) {
> +            done += (*iov)[i].iov_len;
> +            skip -= (*iov)[i].iov_len;
>          } else {
> -            return qemu_co_send(fd, buffer, size);
> +            done += skip;
> +            oldiov->iov_base = (*iov)[i].iov_base;
> +            oldiov->iov_len = (*iov)[i].iov_len;
> +            (*iov)[i].iov_len -= skip;
> +            (*iov)[i].iov_base += skip;
> +            *iov = *iov + i;
> +            *niov = *niov - i;
> +            break;
>          }
>      }

Can you use iov_discard_front instead?

> 
> +                /* XXX see about using qio_channel_yield() in
> +                 * future if we can use normal watches instead
> +                 * of the AIO handlers */
> +                qemu_coroutine_yield();

This breaks dataplane.  The simplest solution is to change
qio_channel_yield() to just

    qio_channel_aio_yield(ioc, qemu_get_aio_context(), condition);

where qio_channel_aio_yield(QIOChannel*, AioContext *, int) is
implemented by the backends.  The implementation could then use
aio_set_fd_handler, and it would be almost trivial because it would
reuse common code just like the one in io/channel-watch.c.

Websock is the only complicated case.  I think it's fine if you simply
must not use qio_channel_{,aio_}yield() for websock channels.

That said, I'm not sure you can even rely on nbd_negotiate_continue to
run in the main I/O thread.  So, another possibility is to add an
alternative watch API similar to AioContext's:

    qio_channel_set_handler(QIOChannel*, AioContext*, void(*in)(void*),
                            void(*out)(void*), void *opaque)

whose implementation would again be simple for backends other than
websock, and probably trivial for backends other than buffer and
websock.  Again, it wouldn't be a problem to skip buffer and websock for
the time being (read, until someone actually needs it).

Converting the block layer to use GMainLoop, unfortunately, would lose
too much performance.

Paolo