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Date:   Mon, 15 Aug 2022 14:28:39 -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: <20220815212839.aop6wwx4fkngihbf@awork3.anarazel.de>
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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.  I'd bet that it also would
fix 6.0rc1, but I'm running out of time to test that.

Greetings,

Andres Freund