Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Feedback-ID: id4a34324:Fastmail
Date:   Mon, 15 Aug 2022 14:46:04 -0700
From:   Andres Freund <andres@anarazel.de>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Guenter Roeck <linux@roeck-us.net>, linux-kernel@vger.kernel.org,
        Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
        Jason Wang <jasowang@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jens Axboe <axboe@kernel.dk>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Greg KH <gregkh@linuxfoundation.org>, c@redhat.com
Subject: Re: [PATCH] virtio_net: Revert "virtio_net: set the default max ring
 size by find_vqs()"
Message-ID: <20220815214604.x7g342h3oadruxx2@awork3.anarazel.de>
References: <20220815090521.127607-1-mst@redhat.com>
 <20220815203426.GA509309@roeck-us.net>
 <20220815164013-mutt-send-email-mst@kernel.org>
 <20220815205053.GD509309@roeck-us.net>
 <20220815165608-mutt-send-email-mst@kernel.org>
 <20220815212839.aop6wwx4fkngihbf@awork3.anarazel.de>
 <20220815173256-mutt-send-email-mst@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20220815173256-mutt-send-email-mst@kernel.org>
Precedence: bulk

Hi,

On 2022-08-15 17:39:08 -0400, Michael S. Tsirkin wrote:
> On Mon, Aug 15, 2022 at 02:28:39PM -0700, Andres Freund wrote:
> > 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 ...

Yea, I meant that added both vp_legacy_set_queue_size() and a call to it. I
was just quickly experimenting around.


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

Cool.


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

Makes sense.

Greetings,

Andres Freund