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

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Date:   Mon, 17 Apr 2023 11:51:22 +0000
From:   Alvaro Karsz <alvaro.karsz@...id-run.com>
To:     "Michael S. Tsirkin" <mst@...hat.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

> > 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?

At the moment, most of the commands chain 3 descriptors:
1 - class + command
2 - command specific
3 - ack

We could merge 1 and 2 into a single one, both are read only for the device, so I take it back, it won't need a bigger ring.
But it will need 2 descriptors at least(1 read only for the device and 1 write only for the device), so we still need to fail probe sometimes.