linux-kernel - Re: [PATCH v3 3/3] vhost: Improve vhost_get_avail_idx() with smp

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Date: Mon, 8 Apr 2024 03:33:19 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Gavin Shan <gshan@...hat.com>
Cc: virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org,
	jasowang@...hat.com, will@...nel.org, davem@...emloft.net,
	stefanha@...hat.com, sgarzare@...hat.com, keirf@...gle.com,
	yihyu@...hat.com, shan.gavin@...il.com
Subject: Re: [PATCH v3 3/3] vhost: Improve vhost_get_avail_idx() with
 smp_rmb()

On Mon, Apr 08, 2024 at 02:15:24PM +1000, Gavin Shan wrote:
> Hi Michael,
> 
> On 3/30/24 19:02, Gavin Shan wrote:
> > On 3/28/24 19:31, Michael S. Tsirkin wrote:
> > > On Thu, Mar 28, 2024 at 10:21:49AM +1000, Gavin Shan wrote:
> > > > All the callers of vhost_get_avail_idx() are concerned to the memory
> > > > barrier, imposed by smp_rmb() to ensure the order of the available
> > > > ring entry read and avail_idx read.
> > > > 
> > > > Improve vhost_get_avail_idx() so that smp_rmb() is executed when
> > > > the avail_idx is advanced. With it, the callers needn't to worry
> > > > about the memory barrier.
> > > > 
> > > > Suggested-by: Michael S. Tsirkin <mst@...hat.com>
> > > > Signed-off-by: Gavin Shan <gshan@...hat.com>
> > > 
> > > Previous patches are ok. This one I feel needs more work -
> > > first more code such as sanity checking should go into
> > > this function, second there's actually a difference
> > > between comparing to last_avail_idx and just comparing
> > > to the previous value of avail_idx.
> > > I will pick patches 1-2 and post a cleanup on top so you can
> > > take a look, ok?
> > > 
> > 
> > Thanks, Michael. It's fine to me.
> > 
> 
> A kindly ping.
> 
> If it's ok to you, could you please merge PATCH[1-2]? Our downstream
> 9.4 need the fixes, especially for NVidia's grace-hopper and grace-grace
> platforms.
> 
> For PATCH[3], I also can help with the improvement if you don't have time
> for it. Please let me know.
> 
> Thanks,
> Gavin

The thing to do is basically diff with the patch I wrote :)
We can also do a bit more cleanups on top of *that*, like unifying
error handling.

-- 
MST