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

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Message-ID: <AM0PR04MB4723F3E6AE381AEC36D1AEFED49C9@AM0PR04MB4723.eurprd04.prod.outlook.com>
Date:   Mon, 17 Apr 2023 07:33:58 +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

> > > > > Actually, I think that all you need to do is disable NETIF_F_SG,
> > > > > and things will work, no?
> > > >
> > > > I think that this is not so simple, if I understand correctly, by disabling NETIF_F_SG we will never receive a chained skbs to transmit, but we still have more functionality to address, for example:
> > > > * The TX timeouts.
> > >
> > > I don't get it. With a linear skb we can transmit it as long as there's
> > > space for 2 entries in the vq: header and data. What's the source of the
> > > timeouts?
> > >
> >
> > I'm not saying that this is not possible, I meant that we need more changes to virtio-net.
> > The source of the timeouts is from the current implementation of virtnet_poll_tx.
> >
> > if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
> >       netif_tx_wake_queue(txq);
> 
> Oh right. So this should check NETIF_F_SG then.
> BTW both ring size and s/g can be tweaked by ethtool, also
> needs handling.
> 

Good point.

> >
> > > > * Guest GSO/big MTU (without VIRTIO_NET_F_MRG_RXBUF?), we can't chain page size buffers anymore.
> > >
> > > I think we can.  mergeable_min_buf_len will just be large.
> > >
> >
> > I meant that we can't just by clearing NETIF_F_SG, we'll need to change virtio-net a little bit more, for example, the virtnet_set_big_packets function.
> >
> 
> Right - for RX, big_packets_num_skbfrags ignores ring size and that's
> probably a bug if mtu is very large.
> 

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.