Re: [virtio-comment] RE: [virtio-dev] RE: [virtio-comment] [PATCH v2] virtio-net: support setting coalescing params for multiple vqs

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jan 22, 2024 at 05:03:38AM +0000, Parav Pandit wrote:
> > >>> The right test on Linux to do without rtnl lock which is anyway ugly
> > >>> and
> > >> wrong semantic to use blocking the whole netdev stack.
> > >>> (in case if you used that).
> > >> Do you have any good directions and attempts to remove rtnl_lock?
> > >>
> > > I think per device lock instead of rtnl is first step that we can start with.
> > 
> Wil check internally who if someone already started working on it.

I feel the issue is at the conceptual level. Yes some drivers will take
a command and just queue it for execution later, but this means that
errors can not be propagated back at all. Imagine device with mac
0x123 in promisc mode. Now commands:

1- program MAC 0xabcdef
2- disable promisc mode

If command 1 fails but 2 proceeds then packets with MAC 0xabc
will be dropped.

Any attempts to batch arbitrary commands will have this issue -
be it at driver or device level.

So, here's my question: what exactly is the guest behaviour
that is driving this work? Is it with a linux guest? which
commands does userspace issue that we need to send multiple
vq coalescing commands? If all you want is to send
same config to all VQs then why not just use
VIRTIO_NET_CTRL_NOTF_COAL_RX_SET as opposed to
VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET ?


-- 
MST


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