Re: [PATCH v3 2/6] xen: pci: introduce reference counting for pdev

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, Apr 17, 2023 at 12:34:31PM +0200, Jan Beulich wrote:
> On 17.04.2023 12:17, Roger Pau Monné wrote:
> > On Fri, Apr 14, 2023 at 01:30:39AM +0000, Volodymyr Babchuk wrote:
> >> Above I have proposed another view on this. I hope, it will work for
> >> you. Just to reiterate, idea is to allow "harmless" refcounts to be left
> >> after returning from pci_remove_device(). By "harmless" I mean that
> >> owners of those refcounts will not try to access the physical PCI
> >> device if pci_remove_device() is already finished.
> > 
> > I'm not strictly a maintainer of this piece code, albeit I have an
> > opinion.  I will like to also hear Jans opinion, since he is the
> > maintainer.
> 
> I'm afraid I can't really appreciate the term "harmless refcounts". Whoever
> holds a ref is entitled to access the device. As stated before, I see only
> two ways of getting things consistent: Either pci_remove_device() is
> invoked upon dropping of the last ref,

With this approach, what would be the implementation of
PHYSDEVOP_manage_pci_remove?  Would it just check whether the pdev
exist and either return 0 or -EBUSY?

> or it checks that it is dropping the
> last one. The former looks architecturally cleaner to me, but I can accept
> that moving there might be more of a change, so wouldn't object to going
> the latter route.

One of my concerns is what is expected of PHYSDEVOP_manage_pci_remove,
I don't think it's expected for PHYSDEVOP_manage_pci_remove to return
0 while there are users inside the hypervisor still holding a
reference to the pdev.

Thanks, Roger.