Re: [PATCH v12.2 01/15] vpci: use per-domain PCI lock to protect vpci structure

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Jan 25, 2024 at 12:23:05PM +0100, Jan Beulich wrote:
> On 25.01.2024 10:05, Roger Pau Monné wrote:
> > On Thu, Jan 25, 2024 at 08:43:05AM +0100, Jan Beulich wrote:
> >> On 24.01.2024 18:51, Roger Pau Monné wrote:
> >>> On Wed, Jan 24, 2024 at 12:34:10PM +0100, Jan Beulich wrote:
> >>>> On 24.01.2024 10:24, Roger Pau Monné wrote:
> >>>>> On Wed, Jan 24, 2024 at 09:48:35AM +0100, Jan Beulich wrote:
> >>>>>> On 23.01.2024 16:07, Roger Pau Monné wrote:
> >>>>>>> On Tue, Jan 23, 2024 at 03:32:12PM +0100, Jan Beulich wrote:
> >>>>>>>> On 15.01.2024 20:43, Stewart Hildebrand wrote:
> >>>>>>>>> @@ -2888,6 +2888,8 @@ int allocate_and_map_msi_pirq(struct domain 
> >>>>>>>>> *d, int index, int *pirq_p,
> >>>>>>>>>  {
> >>>>>>>>>      int irq, pirq, ret;
> >>>>>>>>>  
> >>>>>>>>> +    ASSERT(pcidevs_locked() || rw_is_locked(&d->pci_lock));
> >>>>>>>>
> >>>>>>>> If either lock is sufficient to hold here, ...
> >>>>>>>>
> >>>>>>>>> --- a/xen/arch/x86/physdev.c
> >>>>>>>>> +++ b/xen/arch/x86/physdev.c
> >>>>>>>>> @@ -123,7 +123,9 @@ int physdev_map_pirq(domid_t domid, int type, 
> >>>>>>>>> int *index, int *pirq_p,
> >>>>>>>>>  
> >>>>>>>>>      case MAP_PIRQ_TYPE_MSI:
> >>>>>>>>>      case MAP_PIRQ_TYPE_MULTI_MSI:
> >>>>>>>>> +        pcidevs_lock();
> >>>>>>>>>          ret = allocate_and_map_msi_pirq(d, *index, pirq_p, type, 
> >>>>>>>>> msi);
> >>>>>>>>> +        pcidevs_unlock();
> >>>>>>>>>          break;
> >>>>>>>>
> >>>>>>>
> >>>>>>> IIRC (Stewart can further comment) this is done holding the pcidevs
> >>>>>>> lock to keep the path unmodified, as there's no need to hold the
> >>>>>>> per-domain rwlock.
> >>>>>>
> >>>>>> Yet why would we prefer to acquire a global lock when a per-domain one
> >>>>>> suffices?
> >>>>>
> >>>>> I was hoping to introduce less changes, specially if they are not
> >>>>> strictly required, as it's less risk.  I'm always quite worry of
> >>>>> locking changes.
> >>>>
> >>>> In which case more description / code commenting is needed. The pattern
> >>>> of the assertions looks dangerous.
> >>>
> >>> Is such dangerousness perception because you fear some of the pcidevs
> >>> lock usage might be there not just for preventing the pdev from going
> >>> away, but also to guarantee exclusive access to certain state?
> >>
> >> Indeed. In my view the main purpose of locks is to guard state. Their
> >> use here to guard against devices here is imo rather an abuse; as
> >> mentioned before this should instead be achieved e.g via refcounting.
> >> And it's bad enough already that pcidevs_lock() alone has been abused
> >> this way, without proper marking (leaving us to guess in many places).
> >> It gets worse when a second lock can now also serve this same
> >> purpose.
> > 
> > The new lock is taken in read mode in most contexts, and hence can't
> > be used to indirectly gain exclusive access to domain related
> > structures in a safe way.
> 
> Oh, right - I keep being misled by rw_is_locked(). This is a fair
> argument. Irrespective it would feel better to me if an abstraction
> construct was introduced; but seeing you don't like the idea I guess
> I won't insist.

TBH I'm not going to argue against it if you and Stewart think it's
clearer, but I also won't request the addition of such wrapper myself.

Thanks, Roger.