Re: [PATCH 0/3] vhost cleanups and separate module

[Rusty Russell <rusty@rustcorp.com.au>]

"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Mon, May 06, 2013 at 03:41:36PM +0930, Rusty Russell wrote:
>> Asias He <asias@redhat.com> writes:
>> > Asias He (3):
>> >   vhost: Remove vhost_enable_zcopy in vhost.h
>> >   vhost: Move VHOST_NET_FEATURES to net.c
>> >   vhost: Make vhost a separate module
>> 
>> I like these cleanups, MST pleasee apply.
>
> Absolutely. Except it's 3.11 material and I can only
> usefully create a -next branch once -rc1 is out.
>
>> I have some other cleanups which are on hold for the moment pending
>> MST's vhost_net simplification.  MST, how's that going?
>
> Not too well. The array of status bytes which was designed to complete
> packets in order turns out to be a very efficient datastructure:
>
> It gives us a way to signal completions that is completely lockless for
> multiple completers, and using the producer/consumer model saves extra
> scans for the common case.
>
> Overall I can save some memory and clean up some code but can't get rid
> of the producer/consumer idices (currently named upend/done indices)
> which is what you asked me to do.
> Your cleanups basically don't work with zcopy because they
> ignore the upend/done indices?
> Would you like to post them, noting they only work with zcopy off, and
> we'll look for a way to apply them, together?

Not quite; it's just that I don't understand that code.  It seemed to be
achieving something (ordered completion) which was entirely unnecessary,
so I went on with other things while you removed it.  Now that's not
possible, I'll revisit.

AFAICT we should always do zero copy.  Though I do wonder if we should
use a dedicated hook to get an skb into the tun driver and generate it
ourselves, rather than going sg -> iov -> skb.

Cheers,
Rusty.