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

[Jan Beulich <jbeulich@xxxxxxxx>]

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?

Jan