linux-kernel - Re: [PATCH 4/4] virtio_ring: unify direct/indirect code paths.

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Date:	Thu, 29 May 2014 14:33:17 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>,
	Jens Axboe <axboe@...nel.dk>, Minchan Kim <minchan@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>, "H. Peter Anvin" <hpa@...or.com>,
	Ingo Molnar <mingo@...nel.org>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 4/4] virtio_ring: unify direct/indirect code paths.

On Thu, May 29, 2014 at 08:35:58PM +0930, Rusty Russell wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> > On Thu, May 29, 2014 at 04:56:45PM +0930, Rusty Russell wrote:
> >> Before:
> >> 	gcc 4.8.2: virtio_blk: stack used = 392
> >> 	gcc 4.6.4: virtio_blk: stack used = 480
> >> 
> >> After:
> >> 	gcc 4.8.2: virtio_blk: stack used = 408
> >> 	gcc 4.6.4: virtio_blk: stack used = 432
> >
> > Is it worth it to make the good compiler worse? People are going to use
> > the newer GCC more as time goes on anyhow.
> 
> No, but it's only 16 bytes of stack loss for a simplicity win:
> 
>  virtio_ring.c |  120 +++++++++++++++++++++-------------------------------------
>  1 file changed, 45 insertions(+), 75 deletions(-)
> 
> Cheers,
> Rusty.

I'm concerned that we are doing an extra descriptor walk now though.
And desc == &vq.desc at the end is kind of ugly too.

How about
		if (indirect)
                        vq->vring.desc[i].next = i + 1;
		else
                        i = vq->vring.desc[i].next;

or something like this?

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