netdev - Re: thoughts stac/clac and get user for vhost

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Message-ID: <a0306f51-269c-73b0-f229-2dbf33ef182c@redhat.com>
Date:   Wed, 26 Dec 2018 12:03:50 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org
Subject: Re: thoughts stac/clac and get user for vhost

On 2018/12/26 上午12:41, Michael S. Tsirkin wrote:
> Hi!
> I was just wondering: packed ring batches things naturally.
> E.g.
>
> user_access_begin
> check descriptor valid
> smp_rmb
> copy descriptor
> user_access_end

But without speculation on the descriptor (which may only work for 
in-order or even a violation of spec). Only one two access of a single 
descriptor could be batched. For split ring, we can batch more since we 
know how many descriptors is pending. (avail_idx - last_avail_idx).

Anything I miss?

Thanks

>
> So packed layout should show the gain with this approach.
> That could be motivation enough to finally enable vhost packed ring
> support.
>
> Thoughts?
>