netdev - Re: [PATCH net] virtio-net: reject small vring sizes

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Message-ID: <20230417073830-mutt-send-email-mst@kernel.org>
Date:   Mon, 17 Apr 2023 07:40:19 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Alvaro Karsz <alvaro.karsz@...id-run.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] virtio-net: reject small vring sizes

On Mon, Apr 17, 2023 at 10:04:56AM +0000, Alvaro Karsz wrote:
> > > So, what do you think, we should fix virtio-net to work with smaller rings? we should fail probe?
> > >
> > > I think that since this never came up until now, there is no big demand to such small rings.
> > 
> > The worry is that once we start failing probe there's just a tiny chance
> > hosts begin to rely on us failing probe then we won't be able to fix it.
> > So it depends on the size of the patch I think. So far it seems small enough
> > that wasting code on failing probe isn't worth it.
> > 
> 
> I see your point.
> Regardless, we'll need to fail probe in some cases.
> ring size of 1 for example (if I'm not mistaken)

Hmm. We can make it work if we increase hard header size, then
there will always be room for vnet header.

> control vq even needs a bigger ring.

Why does it?

> Maybe we can fix virtnet to allow smaller rings + fail probe in some cases, all in the same patch/patchset.

If we can't make it work then yes.

-- 
MST