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Date: Thu, 29 May 2014 14:33:17 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH 4/4] virtio_ring: unify direct/indirect code paths.
Message-ID: <20140529113317.GE30210@redhat.com>
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To: Rusty Russell <rusty@rustcorp.com.au>
Cc: Peter Zijlstra <peterz@infradead.org>, Linus Torvalds <torvalds@linux-foundation.org>, Dave Chinner <david@fromorbit.com>, Jens Axboe <axboe@kernel.dk>, Minchan Kim <minchan@kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Andrew Morton <akpm@linux-foundation.org>, linux-mm <linux-mm@kvack.org>, "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@kernel.org>, Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>, Hugh Dickins <hughd@google.com>, Dave Hansen <dave.hansen@intel.com>, Steven Rostedt <rostedt@goodmis.org>

On Thu, May 29, 2014 at 08:35:58PM +0930, Rusty Russell wrote:
> Peter Zijlstra <peterz@infradead.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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