Re: [PATCH v4 4/5] vpci: add SR-IOV support for PVH Dom0

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.11.2026 14:22, Mykyta Poturai wrote:
> On 10/28/26 3:49 PM, Jan Beulich wrote:
>> On 15.10.2026 16:08, Mykyta Poturai wrote:
>>> @@ -139,6 +142,47 @@ const unsigned long *pci_get_ro_map(u16 seg)
>>>       return pseg ? pseg->ro_map : NULL;
>>>   }
>>>
>>> +/*
>>> + * Marking is serialised by the hardware domain's pci_lock, which the 
>>> SR-IOV
>>> + * control register write handler holds; unmarking only clears a bit.
>>> + */
>>
>> What about pci_vf_setup_pending()? How's that (to be) serialized again 
>> marking
>> and unmarking?
>>
> It isn't. The test runs ahead of any lock because adding the VF takes them.
> 
> We could put it under the hardware domain's pci_lock, which is what
> control_write() holds for marking and unmarking, but it would have to be
> dropped before vpci_sriov_add_vf() anyway: pci_add_device() takes 
> pcidevs_lock,
> and the order is pcidevs_lock then pci_lock. So the test and the add can't be
> made atomic with the locks we have. Is the lock worth taking just for the bit
> test?

No, such locking would make sense only when held from before the test until
after the result was actually acted upon (in the case here: the bit was
cleared by pci_vf_setup_end()).

pcidevs_lock vs d->pci_lock wouldn't really be an issue here, though: The
former is recursive, so you could acquire ahead of acquiring d->pci_lock.
The "inner" acquire in pci_add_device() would then be little more than a
no-op. (Of course proper commentary would need adding if such an approach
was taken.)

> The only thing I can see actually misfiring is vf_setup_map: xvzalloc_array()
> zeroes it and the pointer store publishes it with nothing in between, so a
> reader can theoretically see the pointer ahead of the zeroing and test an
> uninitialised bit. Should we reallocate it in alloc_pseg(), or just add a 
> memory barrier?

Hmm, is this really the only issue? What if two racing pci_vf_setup_pending()
both yield true? Then we'd end up with two competing vpci_sriov_add_vf().

Jan