linux-kernel - Re: virtio-blk: should num_vqs be limited by num_possible

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Date:   Fri, 15 Mar 2019 13:41:12 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Dongli Zhang <dongli.zhang@...cle.com>, mst@...hat.com,
        virtualization@...ts.linux-foundation.org,
        linux-block@...r.kernel.org, axboe@...nel.dk,
        linux-kernel@...r.kernel.org
Subject: Re: virtio-blk: should num_vqs be limited by num_possible_cpus()?

On Fri, 15 Mar 2019 12:50:11 +0800
Jason Wang <jasowang@...hat.com> wrote:

> Or something like I proposed several years ago? 
> https://do-db2.lkml.org/lkml/2014/12/25/169
> 
> Btw, for virtio-net, I think we actually want to go for having a maximum 
> number of supported queues like what hardware did. This would be useful 
> for e.g cpu hotplug or XDP (requires per cpu TX queue). But the current 
> vector allocation doesn't support this which will results all virtqueues 
> to share a single vector. We may indeed need more flexible policy here.

I think it should be possible for the driver to give the transport
hints how to set up their queues/interrupt structures. (The driver
probably knows best about its requirements.) Perhaps whether a queue is
high or low frequency, or whether it should be low latency, or even
whether two queues could share a notification mechanism without
drawbacks. It's up to the transport to make use of that information, if
possible.