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From:   Alvaro Karsz <alvaro.karsz@solid-run.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
CC:     Jason Wang <jasowang@redhat.com>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "edumazet@google.com" <edumazet@google.com>,
        "kuba@kernel.org" <kuba@kernel.org>,
        "pabeni@redhat.com" <pabeni@redhat.com>,
        "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH net] virtio-net: reject small vring sizes
Thread-Topic: [PATCH net] virtio-net: reject small vring sizes
Thread-Index: AQHZcDeGnH5xR2OGlkuo4s6jvhNMC68uIjT9gABGFgCAAG90AIAAMSwAgAABjzCAAARBAIAABATI
Date:   Mon, 17 Apr 2023 07:03:52 +0000
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> > > Actually, I think that all you need to do is disable NETIF_F_SG,=0A=
> > > and things will work, no?=0A=
> >=0A=
> > I think that this is not so simple, if I understand correctly, by disab=
ling NETIF_F_SG we will never receive a chained skbs to transmit, but we st=
ill have more functionality to address, for example:=0A=
> > * The TX timeouts.=0A=
> =0A=
> I don't get it. With a linear skb we can transmit it as long as there's=
=0A=
> space for 2 entries in the vq: header and data. What's the source of the=
=0A=
> timeouts?=0A=
> =0A=
=0A=
I'm not saying that this is not possible, I meant that we need more changes=
 to virtio-net.=0A=
The source of the timeouts is from the current implementation of virtnet_po=
ll_tx.=0A=
=0A=
if (sq->vq->num_free >=3D 2 + MAX_SKB_FRAGS)=0A=
	netif_tx_wake_queue(txq);=0A=
=0A=
=0A=
> > * Guest GSO/big MTU (without VIRTIO_NET_F_MRG_RXBUF?), we can't chain p=
age size buffers anymore.=0A=
> =0A=
> I think we can.  mergeable_min_buf_len will just be large.=0A=
> =0A=
=0A=
I meant that we can't just by clearing NETIF_F_SG, we'll need to change vir=
tio-net a little bit more, for example, the virtnet_set_big_packets functio=
n.=0A=