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

[Jan Beulich <jbeulich@xxxxxxxx>]

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. Even if (as you say in a later reply)
this is only temporary, we all know how long "temporary" can be. It
might even be advisable to introduce a helper construct. That would then
be where the respective code comment goes, clarifying that the _sole_
purpose is to guarantee a pdev to not go away; no further protection of
any state, no other critical region aspects (assuming of course all of
this is actually true for all of the affected use sites).

Jan