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

[Anthony Liguori <aliguori@us.ibm.com>]

On 08/04/2011 11:04 AM, Avi Kivity wrote:
> 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?

You poll by trying to write.  If write fails, you can set a callback for 
notification for when it becomes writable.

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

This series won't block the vcpu.  The devices can (and will) register 
callbacks for when the pipe is writable again.

Regards,

Anthony Liguori

>