Re: About the instance_finalize callback in VFIO PCI

[Yang Zhong]

On Wed, Mar 22, 2023 at 12:22:27PM -0600, Alex Williamson wrote:
> On Wed, 22 Mar 2023 09:10:20 -0400
> Yang Zhong <yang.zhong@linux.intel.com> wrote:
> 
> > On Wed, Mar 22, 2023 at 01:56:13PM +0100, Cédric Le Goater wrote:
> > > On 3/22/23 13:28, Yang Zhong wrote:  
> > > > On Tue, Mar 21, 2023 at 06:30:14PM +0100, Cédric Le Goater wrote:  
> > > > > On 3/20/23 10:31, Yang Zhong wrote:  
> > > > > > Hello Alex and Paolo,
> > > > > > 
> > > > > > There is one instance_finalize callback definition in 
> > > > > > hw/vfio/pci.c, but
> > > > > > i find this callback(vfio_instance_finalize()) never be called 
> > > > > > during the
> > > > > > VM shutdown with close VM or "init 0" command in guest.
> > > > > > 
> > > > > > The Qemu related command:
> > > > > >      ......
> > > > > >      -device vfio-pci,host=d9:00.0
> > > > > >      ......  
> > > > > 
> > > > > well, the finalize op is correctly called for hot unplugged devices, 
> > > > > using
> > > > > device_add.
> > > > >   
> > > >     Thanks Cédric, i can use device_del command in the monitor to
> > > >     trigger this instance_finalize callback function in the VFIO PCI.
> > > >     thanks!  
> > > 
> > > yes. I think that in the shutdown case, QEMU simply relies on exit() to
> > > do the cleanup. On the kernel side, unmaps, fds being closed trigger any
> > > allocated resources.
> > > 
> > > Out of curiosity, what were you trying to achieve in the finalize op ?
> > >   
> >  
> >  We are doing one new feature, which need this callback to do some
> >  cleanup work with VFIO/iommufd kernel module. thanks!
> 
> This sounds dangerously like relying on userspace for cleanup.  Kernel
> drivers need to be able to perform all cleanup themselves when file
> descriptors are closed.  They must expect that userspace can be killed
> at any point in time w/o an opportunity to do cleanup work.  Thanks,
> 

  Thanks Alex, yes, we have moved these cleanup to kernel driver side.
  I was just curious about what scenario this instance_finalize callback 
  is used in VFIO PCI, now it is clear, thanks a lot!

  Regards,
  Yang


> Alex
>