netdev - Re: upstream kernel crashes

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Date:   Mon, 15 Aug 2022 14:10:35 -0700
From:   Andres Freund <andres@...razel.de>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     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>,
        Guenter Roeck <linux@...ck-us.net>,
        linux-kernel@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>,
        c@...hat.com
Subject: Re: upstream kernel crashes

Hi,

On 2022-08-15 14:04:37 -0700, Andres Freund wrote:
> Booting with the equivalent change, atop 5.19, in the legacy setup_vq()
> reliably causes boot to hang:

I don't know much virtio, so take this with a rock of salt:

Legacy setup_vq() doesn't tell the host about the queue size. The modern one
does:
	vp_modern_set_queue_size(mdev, index, virtqueue_get_vring_size(vq));
but the legacy one doesn't.

I assume this means the host will assume the queue is of the size suggested by
vp_legacy_get_queue_size(). If the host continues to write into the space
after the "assumed end" of the queue, but the guest puts other stuff in that
space, well, I'd expect fun roughly like the stuff we've been seeing in this
and related threads.

Greetings,

Andres Freund