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

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Date:   Sun, 23 Apr 2023 12:28:49 +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


> > > The rest of stuff can probably just be moved to after find_vqs without
> > > much pain.
> > >
> > Actually, I think that with a little bit of pain :)
> > If we use small vrings and a GRO feature bit is set, Linux will need to allocate 64KB of continuous memory for every receive descriptor..
> 
> Oh right. Hmm. Well this is same as big packets though, isn't it?
> 

Well, when VIRTIO_NET_F_MRG_RXBUF is not negotiated and one of the GRO features is, the receive buffers are page size buffers chained together to form a 64K buffer.
In this case, do all the chained descriptors actually point to a single block of continuous memory, or is it possible for the descriptors to point to pages spread all over?

> 
> > Instead of failing probe if GRO/CVQ are set, can we just reset the device if we discover small vrings and start over?
> > Can we remember that this device uses small vrings, and then just avoid negotiating the features that we cannot support?
> 
> 
> We technically can of course. I am just not sure supporting CVQ with just 1 s/g entry will
> ever be viable.

Even if we won't support 1 s/g entry, do we want to fail probe in such cases?
We could just disable the CVQ feature (with reset, as suggested before).
I'm not saying that we should, just raising the option.