Re: [PATCH v8 02/13] vpci: use per-domain PCI lock to protect vpci structure

[Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>]

Hi Jan

Jan Beulich <jbeulich@suse.com> writes:

> On 27.07.2023 02:56, Volodymyr Babchuk wrote:
>> Hi Roger,
>> 
>> Roger Pau Monné <roger.pau@citrix.com> writes:
>> 
>>> On Wed, Jul 26, 2023 at 01:17:58AM +0000, Volodymyr Babchuk wrote:
>>>>
>>>> Hi Roger,
>>>>
>>>> Roger Pau Monné <roger.pau@citrix.com> writes:
>>>>
>>>>> On Thu, Jul 20, 2023 at 12:32:31AM +0000, Volodymyr Babchuk wrote:
>>>>>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>>>>>> @@ -498,6 +537,7 @@ void vpci_write(pci_sbdf_t sbdf, unsigned int reg, unsigned int size,
>>>>>>          ASSERT(data_offset < size);
>>>>>>      }
>>>>>>      spin_unlock(&pdev->vpci->lock);
>>>>>> +    unlock_locks(d);
>>>>>
>>>>> There's one issue here, some handlers will cal pcidevs_lock(), which
>>>>> will result in a lock over inversion, as in the previous patch we
>>>>> agreed that the locking order was pcidevs_lock first, d->pci_lock
>>>>> after.
>>>>>
>>>>> For example the MSI control_write() handler will call
>>>>> vpci_msi_arch_enable() which takes the pcidevs lock.  I think I will
>>>>> have to look into using a dedicated lock for MSI related handling, as
>>>>> that's the only place where I think we have this pattern of taking the
>>>>> pcidevs_lock after the d->pci_lock.
>>>>
>>>> I'll mention this in the commit message. Is there something else that I
>>>> should do right now?
>>>
>>> Well, I don't think we want to commit this as-is with a known lock
>>> inversion.
>>>
>>> The functions that require the pcidevs lock are:
>>>
>>> pt_irq_{create,destroy}_bind()
>>> unmap_domain_pirq()
>>>
>>> AFAICT those functions require the lock in order to assert that the
>>> underlying device doesn't go away, as they do also use d->event_lock
>>> in order to get exclusive access to the data fields.  Please double
>>> check that I'm not mistaken.
>> 
>> You are right, all three function does not access any of PCI state
>> directly. However...
>> 
>>> If that's accurate you will have to check the call tree that spawns
>>> from those functions in order to modify the asserts to check for
>>> either the pcidevs or the per-domain pci_list lock being taken.
>> 
>> ... I checked calls for PT_IRQ_TYPE_MSI case, there is only call that
>> bothers me: hvm_pi_update_irte(), which calls IO-MMU code via
>> vmx_pi_update_irte():
>> 
>> amd_iommu_msi_msg_update_ire() or msi_msg_write_remap_rte().
>> 
>> Both functions read basic pdev fields like sbfd or type. I see no
>> problem there, as values of those fields are not supposed to be changed.
>
> But whether fields are basic or will never change doesn't matter when
> the pdev struct itself suddenly disappears.

This is not a problem, as it is expected that d->pci_lock is being held,
so pdev structure will not disappear. I am trying to answer another
question: is d->pci_lock enough or pcidevs_lock is also should required?

-- 
WBR, Volodymyr