linux-kernel - Re: [PATCH] Bluetooth: virtio

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Date:   Thu, 25 Nov 2021 16:23:12 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Marcel Holtmann <marcel@...tmann.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>,
        linux-bluetooth <linux-bluetooth@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] Bluetooth: virtio_bt: fix device removal

On Thu, Nov 25, 2021 at 10:01:25PM +0100, Marcel Holtmann wrote:
> Hi Michael,
> 
> >>> Device removal is clearly out of virtio spec: it attempts to remove
> >>> unused buffers from a VQ before invoking device reset. To fix, make
> >>> open/close NOPs and do all cleanup/setup in probe/remove.
> >> 
> >> so the virtbt_{open,close} as NOP is not really what a driver is suppose
> >> to be doing. These are transport enable/disable callbacks from the BT
> >> Core towards the driver. It maps to a device being enabled/disabled by
> >> something like bluetoothd for example. So if disabled, I expect that no
> >> resources/queues are in use.
> >> 
> >> Maybe I misunderstand the virtio spec in that regard, but I would like
> >> to keep this fundamental concept of a Bluetooth driver. It does work
> >> with all other transports like USB, SDIO, UART etc.
> >> 
> >>> The cost here is a single skb wasted on an unused bt device - which
> >>> seems modest.
> >> 
> >> There should be no buffer used if the device is powered off. We also don’t
> >> have any USB URBs in-flight if the transport is not active.
> >> 
> >>> NB: with this fix in place driver still suffers from a race condition if
> >>> an interrupt triggers while device is being reset. Work on a fix for
> >>> that issue is in progress.
> >> 
> >> In the virtbt_close() callback we should deactivate all interrupts.
> >> 
> > 
> > If you want to do that then device has to be reset on close,
> > and fully reinitialized on open.
> > Can you work on a patch like that?
> > Given I don't have the device such a rework is probably more
> > than I can undertake.
> 
> so you mean move virtio_find_vqs() into virtbt_open() and del_vqs() into
> virtbt_close()?

And reset before del_vqs.

> Or is there are way to set up the queues without starting them?
> 
> However I am failing to understand your initial concern, we do reset()
> before del_vqs() in virtbt_remove(). Should we be doing something different
> in virtbt_close() other than virtqueue_detach_unused_buf(). Why would I
> keep buffers attached if they are not used.
> 
> Regards
> 
> Marcel

They are not used at that point but until device is reset can use them.
Also, if you then proceed to open without a reset, and kick,
device will start by processing the original buffers, crashing
or corrupting memory.

-- 
MST