linux-kernel - Re: [PATCH 08/16] virtio_ring: virtqueue_add_outbuf / virtqueue_add

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Message-ID: <87sj4pfc61.fsf@rustcorp.com.au>
Date:	Fri, 22 Feb 2013 10:32:46 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>, Asias He <asias@...hat.com>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 08/16] virtio_ring: virtqueue_add_outbuf / virtqueue_add_inbuf.

"Michael S. Tsirkin" <mst@...hat.com> writes:
> On Tue, Feb 19, 2013 at 06:26:26PM +1030, Rusty Russell wrote:
>> These are specialized versions of virtqueue_add_buf(), which cover
>> over 50% of cases and are far clearer.
>> 
>> In particular, the scatterlists passed to these functions don't have
>> to be clean (ie. we ignore end markers).
>> 
>> FIXME: I'm not sure about the unclean sglist bit.  I had a more
>> ambitious one which conditionally ignored end markers in the iterator,
>> but it was ugly and I suspect this is just as fast.  Maybe we should
>> just fix all the drivers?
>> 
>> Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
>
> Looking at code, it seems that most users really have a single sg, in
> low memory. So how about simply passing void * instead of sg? Whoever
> has multiple sgs can use the rich interface.

Good point, let's do that:
1) Make virtqueue_add_outbuf()/inbuf() take a void * and len.
2) Transfer users across to use that.
3) Make everyone else use clean scatterlists with virtqueue_add_sgs[].
4) Remove virtqueue_add_bufs().

> Long term we might optimize this unrolling some loops, I think
> I saw this giving a small performance gain for -net.

I *think* we could make virtqueue_add() an inline and implement an
virtqueue_add_outsg() wrapper and gcc will eliminate the loops for us.
But not sure it's worth the text bloat...

Cheers,
Rusty.
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