Re: [PATCH 2/2] virtio-blk: Ensure no requests in virtqueues before deleting vqs.

[yi sun]

Yes, I'm using virtio-mmio.

On Tue, Jan 23, 2024 at 11:09 PM Stefan Hajnoczi <stefanha@xxxxxxxxxx> wrote:
>
> Hi Michael,
> This could potentially affect other VIRTIO drivers too. Please see
> below.
>
> On Tue, Jan 23, 2024 at 11:27:40AM +0800, yi sun wrote:
> > On Mon, Jan 22, 2024 at 11:43 PM Stefan Hajnoczi <stefanha@xxxxxxxxxx> wrote:
> > >
> > > On Mon, Jan 22, 2024 at 07:07:22PM +0800, Yi Sun wrote:
> > > > Ensure no remaining requests in virtqueues before resetting vdev and
> > > > deleting virtqueues. Otherwise these requests will never be completed.
> > > > It may cause the system to become unresponsive. So it is better to place
> > > > blk_mq_quiesce_queue() in front of virtio_reset_device().
> > >
> > > QEMU's virtio-blk device implementation completes all requests during
> > > device reset. Most device implementations have to do the same to avoid
> > > leaving dangling requests in flight across device reset.
> > >
> > > Which device implementation are you using and why is it safe for the
> > > device is simply drop requests across device reset?
> > >
> > > Stefan
> >
> > Virtio-blk device implementation completes all requests during device reset, but
> > this can only ensure that the device has finished using virtqueue. We should
> > also consider the driver's use of virtqueue.
> >
> > I caught such an example. Before del_vqs, the request had been processed by
> > the device, but it had not been processed by the driver. Although I am
> > using kernel5.4,
> > I think this problem may also occur with the latest version of kernel.
> >
> > The debug code I added is as follows:
> > virtblk_freeze()
> > {
> >         vdev reset();
> >         quiesce queue();
> >         if (virtqueue->num_free != 1024) //1024 is the init value.
> >                 BUG_ON(1);
> >         vdev del_vqs();
> > }
> >
> > BUG_ON triggered the dump, the analysis is as follows:
> >
> > There is one request left in request_queue.
> > crash_arm64> struct request ffffff81f0560000 | grep -e state -e __data_len
> >   __data_len = 20480,
> >   state = MQ_RQ_IN_FLIGHT,
> >
> > crash_arm64> vring_virtqueue.packed,last_used_idx,broken,vq 0xffffff8086f92900 |
> > grep -e num -e used_wrap_counter -e last_used_idx -e broken -e
> > num_free -e desc_state -e "desc ="
> >         num = 1024,
> >         desc = 0xffffff8085ff8000,
> >       used_wrap_counter = false,
> >       desc_state = 0xffffff8085610000,
> >   last_used_idx = 487,
> >   broken = false,
> >     num_free = 1017,
> >
> > Find desc based on last_used_idx. Through flags, we can know that the request
> > has been processed by the device, but it is still in flight state
> > because it has not
> > had time to run virtblk_done().
> > crash_arm> vring_packed_desc ffffff8085ff9e70
> > struct vring_packed_desc {
> >   addr = 10474619192,
> >   len = 20481,
> >   id = 667,
> >   flags = 2
> > }
> >
> > I'm using a closed source virtual machine, so I can't see the source
> > code for it,
> > but I'm guessing it's similar to qemu.
> >
> > After the device completes the request, we must also ensure that the driver can
> > complete the request in virtblk_done().
> >
>
> Okay, I think your approach of waiting for requests before
> virtio_device_reset() makes sense. blk_mq_complete_request() is async
> (might be deferred to an IPI or softirq) so it's not enough for
> virtblk_done() to run before virtio_device_reset() returns. There is no
> guarantee that virtblk_request_done() runs before virtio_device_reset()
> returns.
>
> A separate issue about virtio_device_reset():
>
> Are you using virtio-mmio? virtio-mmio's vm_reset() doesn't offer the
> same guarantees as virtio-pci's reset functions. virtio-pci guarantees
> that irqs sent by the device during the reset operation complete and the
> irq handlers run before virtio_device_reset() returns. virtio-mmio does
> not.
>
> (On top of this, virtio_device_reset() now has
> CONFIG_VIRTIO_HARDEN_NOTIFICATION which changes the semantics. Drivers
> cannot expect to complete any in-flight requests in
> virtio_device_reset() when complied with this config option.)
>
> Other drivers may be affected by these inconsistent
> virtio_device_reset() semantics. I haven't audited the code.
>
> Stefan