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

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

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.

Thanks, Roger.