linux-kernel - Re: [RFC PATCH net 0/3] virtio-net: allow usage of small vrings

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Message-ID: <AM0PR04MB4723C479C388266434DE415ED4699@AM0PR04MB4723.eurprd04.prod.outlook.com>
Date:   Sun, 30 Apr 2023 18:15:03 +0000
From:   Alvaro Karsz <alvaro.karsz@...id-run.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     "jasowang@...hat.com" <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>,
        "xuanzhuo@...ux.alibaba.com" <xuanzhuo@...ux.alibaba.com>
Subject: Re: [RFC PATCH net 0/3] virtio-net: allow usage of small vrings

> > This patchset follows a discussion in the mailing list [1].
> >
> > This fixes only part of the bug, rings with less than 4 entries won't
> > work.
> 
> Why the difference?
> 

Because the RING_SIZE < 4 case requires much more adjustments.

* We may need to squeeze the virtio header into the headroom.
* We may need to squeeze the GSO header into the headroom, or block the features.
* At the moment, without NETIF_F_SG, we can receive a skb with 2 segments, we may need to reduce it to 1.
* We may need to change all the control commands, so class,  command and command specific data will fit in a single segment.
* We may need to disable the control command and all the features depending on it.
* We may need to disable NAPI?

There may be more changes..

I was thinking that it may be easier to start with the easier case RING_SIZE >= 4, make sure everything is working fine, then send a follow up patchset with the required adjustments for RING_SIZE < 4.