netdev - Re: [PATCH net-next v4] net: virtio_net: notifications coalescing support

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Date:   Wed, 20 Jul 2022 16:28:31 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Alvaro Karsz <alvaro.karsz@...id-run.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next v4] net: virtio_net: notifications coalescing support

On Wed, Jul 20, 2022 at 3:57 PM Alvaro Karsz <alvaro.karsz@...id-run.com> wrote:
>
> > And we need an upper limit for those values, this helps for e.g
> > migration compatibility.
>
> Why not let the device decide the upper limit, making it device specific?

For example we may want to migrate VM from src to dst.

In src, we set tx-max-coalesced-frames to 512 but the dst only supports 256.

> As written in the spec., a device can answer to the coalescing command
> with VIRTIO_NET_ERR,
> If it was not able to set the requested settings.
>
> If a physical device uses virtio datapath, and can for example
> coalesce notifications up to 500us, why should we limit it with a
> lower number?
>

See above, for migration compatibility.

Thanks