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

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Date:   Mon, 17 Apr 2023 02:41:04 -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 06:38:43AM +0000, Alvaro Karsz wrote:
> > 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?

> * 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.


> > Alvaro, can you try?
> 
> It won't matter at the moment, we'll get TX timeout after the first tx packet, we need to address this part as well.