Re: [PATCH] Revert "virtio_pci: Support surprise removal of virtio pci device"

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Aug 22, 2025 at 01:49:36PM +0000, Parav Pandit wrote:
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: 22 August 2025 06:34 PM
> > 
> > On Fri, Aug 22, 2025 at 12:22:50PM +0000, Parav Pandit wrote:
> > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > Sent: 22 August 2025 03:52 PM
> > > >
> > > > On Fri, Aug 22, 2025 at 12:17:06PM +0300, Parav Pandit wrote:
> > > > > This reverts commit 43bb40c5b926 ("virtio_pci: Support surprise
> > > > > removal of
> > > > virtio pci device").
> > > > >
> > > > > Virtio drivers and PCI devices have never fully supported true
> > > > > surprise (aka hot unplug) removal. Drivers historically continued
> > > > > processing and waiting for pending I/O and even continued
> > > > > synchronous device reset during surprise removal. Devices have
> > > > > also continued completing I/Os, doing DMA and allowing device
> > > > > reset after surprise removal to support such drivers.
> > > > >
> > > > > Supporting it correctly would require a new device capability
> > > >
> > > > If a device is removed, it is removed.
> > > This is how it was implemented and none of the virtio drivers supported it.
> > > So vendors had stepped away from such device implementation.
> > > (not just us).
> > 
> > 
> > If the slot does not have a mechanical interlock, I can pull the device out. It's
> > not up to a device implementation.
> 
> Sure yes, stack is not there yet to support it.
> Each of the virtio device drivers are not there yet.
> Lets build that infra, let device indicate it and it will be smooth ride for driver and device.

There is simply no way for the device to "support" for surprise removal,
or lack such support thereof. The support is up to the slot, not the
device.  Any pci compliant device can be placed in a slot that allows
surprise removal and that is all. The user can then remove the device.
Software can then either recover gracefully - it should - or hang or
crash - it does sometimes, now. The patch you are trying to revert
is an attempt to move some use-cases from the 1st to the 2nd category.

But what is going on now, as far as I could tell, is that someone developed
a surprise removal emulation that does not actually remove the device,
and is using that for testing the code in linux that
supports surprise removal.  That weird emulation
seems to lead to all kind of weird issues. You answer is to remove the
existing code and tell your testing team "we do not support surprise removal".

But just go ahead and tell this to them straight away. You do not need
this patch for this.


Or better still, let's fix the issues please.


-- 
MST