netdev - Re: [PATCH] virtio_net: Revert "virtio_net: set the default max ring size by find

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Date:   Mon, 15 Aug 2022 17:39:08 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Andres Freund <andres@...razel.de>
Cc:     Guenter Roeck <linux@...ck-us.net>, linux-kernel@...r.kernel.org,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Greg KH <gregkh@...uxfoundation.org>, c@...hat.com
Subject: Re: [PATCH] virtio_net: Revert "virtio_net: set the default max ring
 size by find_vqs()"

On Mon, Aug 15, 2022 at 02:28:39PM -0700, Andres Freund wrote:
> Hi,
> 
> On 2022-08-15 17:04:10 -0400, Michael S. Tsirkin wrote:
> > So virtio has a queue_size register. When read, it will give you
> > originally the maximum queue size. Normally we just read it and
> > use it as queue size.
> > 
> > However, when queue memory allocation fails, and unconditionally with a
> > network device with the problematic patch, driver is asking the
> > hypervisor to make the ring smaller by writing a smaller value into this
> > register.
> > 
> > I suspect that what happens is hypervisor still uses the original value
> > somewhere.
> 
> It looks more like the host is never told about the changed size for legacy
> devices...
> 
> Indeed, adding a vp_legacy_set_queue_size() & call to it to setup_vq(), makes
> 5.19 + restricting queue sizes to 1024 boot again.

Interesting, the register is RO in the legacy interface.
And to be frank I can't find where is vp_legacy_set_queue_size
even implemented. It's midnight here too ...

>  I'd bet that it also would
> fix 6.0rc1, but I'm running out of time to test that.
> 
> Greetings,
> 
> Andres Freund

Yes I figured this out too. And I was able to reproduce on qemu now.

Andres thanks a lot for the help!

I'm posting a new patchset reverting all the handing of resize
restrictions, I think we should rethink it for the next release.

Thanks everyone for the help!

-- 
MST