Re: [Qemu-devel] [PATCH 03/12] char: introduce tx queue to enable Unix style flow control

[Avi Kivity <avi@redhat.com>]

On 08/01/2011 05:23 PM, Anthony Liguori wrote:
> The char layer tries very hard to avoid using an intermediate buffer.  The
> implication of this is that when the backend does a write(), the data for that
> write must be immediately passed to the front end.
>
> Flow control is needed to handle the likely event that the front end is not
> able to handle the data at this point in time.  We implement flow control
> today by allowing the front ends to register a polling function.  The polling
> function returns non-zero when it is able to receive data.
>
> This works okay because most backends are tied to some sort of file descriptor
> and our main loop allows polling to be included with file descriptor
> registration.
>
> This falls completely apart when dealing with the front end writing to the
> back end though because the front end (devices) don't have an obvious place to
> integrate polling.
>
> Short summary: we're broken by design.  A way to fix this is to eliminate
> polling entirely and use a Unix style flow control mechanism.  This involves
> using an intermediate buffer and allowing registration of notifications when
> the buffer either has data in it (readability) or is not full (writability).
>

If you don't have an obvious place to integrate polling, how do you poll 
for writability?

Although, providing a reasonably sized buffer and blocking the vcpu when 
it's full is a lot better than what we have now.

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