linux-kernel - Re: [PATCH v2 3/3] tun: Set num

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Message-ID: <1f2908ed-e938-4365-8f1e-9f1c7753fb9b@daynix.com>
Date: Fri, 10 Jan 2025 19:04:07 +0900
From: Akihiko Odaki <akihiko.odaki@...nix.com>
To: Jason Wang <jasowang@...hat.com>
Cc: Jonathan Corbet <corbet@....net>,
 Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 "Michael S. Tsirkin" <mst@...hat.com>, Xuan Zhuo
 <xuanzhuo@...ux.alibaba.com>, Shuah Khan <shuah@...nel.org>,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org, kvm@...r.kernel.org,
 virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org,
 Yuri Benditovich <yuri.benditovich@...nix.com>,
 Andrew Melnychenko <andrew@...nix.com>,
 Stephen Hemminger <stephen@...workplumber.org>, gur.stavi@...wei.com,
 devel@...nix.com
Subject: Re: [PATCH v2 3/3] tun: Set num_buffers for virtio 1.0

On 2025/01/10 12:27, Jason Wang wrote:
> On Thu, Jan 9, 2025 at 2:59 PM Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
>>
>> The specification says the device MUST set num_buffers to 1 if
>> VIRTIO_NET_F_MRG_RXBUF has not been negotiated.
> 
> Have we agreed on how to fix the spec or not?
> 
> As I replied in the spec patch, if we just remove this "MUST", it
> looks like we are all fine?

My understanding is that we should fix the kernel and QEMU instead. 
There may be some driver implementations that assumes num_buffers is 1 
so the kernel and QEMU should be fixed to be compatible with such 
potential implementations.

It is also possible to make future drivers with existing kernels and 
QEMU by ensuring they will not read num_buffers when 
VIRTIO_NET_F_MRG_RXBUF has not negotiated, and that's what "[PATCH v3] 
virtio-net: Ignore num_buffers when unused" does.
https://lore.kernel.org/r/20250110-reserved-v3-1-2ade0a5d2090@daynix.com

Regards,
Akihiko Odaki