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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan