Re: [PATCH] vpci: introduce per-domain lock to protect vpci structure

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>]

On 14.02.22 13:11, Roger Pau Monné wrote:
> On Mon, Feb 14, 2022 at 10:53:43AM +0000, Oleksandr Andrushchenko wrote:
>>
>> On 14.02.22 12:34, Roger Pau Monné wrote:
>>> On Mon, Feb 14, 2022 at 09:36:39AM +0000, Oleksandr Andrushchenko wrote:
>>>> On 11.02.22 13:40, Roger Pau Monné wrote:
>>>>> +
>>>>>>>>          for ( i = 0; i < msix->max_entries; i++ )
>>>>>>>>          {
>>>>>>>>              const struct vpci_msix_entry *entry = &msix->entries[i];
>>>>>>> Since this function is now called with the per-domain rwlock read
>>>>>>> locked it's likely not appropriate to call process_pending_softirqs
>>>>>>> while holding such lock (check below).
>>>>>> You are right, as it is possible that:
>>>>>>
>>>>>> process_pending_softirqs -> vpci_process_pending -> read_lock
>>>>>>
>>>>>> Even more, vpci_process_pending may also
>>>>>>
>>>>>> read_unlock -> vpci_remove_device -> write_lock
>>>>>>
>>>>>> in its error path. So, any invocation of process_pending_softirqs
>>>>>> must not hold d->vpci_rwlock at least.
>>>>>>
>>>>>> And also we need to check that pdev->vpci was not removed
>>>>>> in between or *re-created*
>>>>>>> We will likely need to re-iterate over the list of pdevs assigned to
>>>>>>> the domain and assert that the pdev is still assigned to the same
>>>>>>> domain.
>>>>>> So, do you mean a pattern like the below should be used at all
>>>>>> places where we need to call process_pending_softirqs?
>>>>>>
>>>>>> read_unlock
>>>>>> process_pending_softirqs
>>>>>> read_lock
>>>>>> pdev = pci_get_pdev_by_domain(d, sbdf.seg, sbdf.bus, sbdf.devfn);
>>>>>> if ( pdev && pdev->vpci && is_the_same_vpci(pdev->vpci) )
>>>>>> <continue processing>
>>>>> Something along those lines. You likely need to continue iterate using
>>>>> for_each_pdev.
>>>> How do we tell if pdev->vpci is the same? Jan has already brought
>>>> this question before [1] and I was about to use some ID for that purpose:
>>>> pdev->vpci->id = d->vpci_id++ and then we use pdev->vpci->id  for checks
>>> Given this is a debug message I would be OK with just doing the
>>> minimal checks to prevent Xen from crashing (ie: pdev->vpci exists)
>>> and that the resume MSI entry is not past the current limit. Otherwise
>>> just print a message and move on to the next device.
>> Agree, I see no big issue (probably) if we are not able to print
>>
>> How about this one:
>>
>> diff --git a/xen/drivers/vpci/header.c b/xen/drivers/vpci/header.c
>> index 809a6b4773e1..50373f04da82 100644
>> --- a/xen/drivers/vpci/header.c
>> +++ b/xen/drivers/vpci/header.c
>> @@ -171,10 +171,31 @@ static int __init apply_map(struct domain *d, const 
>> struct pci_dev *pdev,
>>                                struct rangeset *mem, uint16_t cmd)
>>    {
>>        struct map_data data = { .d = d, .map = true };
>> +    pci_sbdf_t sbdf = pdev->sbdf;
>>        int rc;
>>
>> + ASSERT(rw_is_write_locked(&pdev->domain->vpci_rwlock));
>> +
>>        while ( (rc = rangeset_consume_ranges(mem, map_range, &data)) == 
>> -ERESTART )
>> +    {
>> +
>> +        /*
>> +         * process_pending_softirqs may trigger vpci_process_pending which
>> +         * may need to acquire pdev->domain->vpci_rwlock in read mode.
>> +         */
>> +        write_unlock(&pdev->domain->vpci_rwlock);
>>            process_pending_softirqs();
>> +        write_lock(&pdev->domain->vpci_rwlock);
>> +
>> +        /* Check if pdev still exists and vPCI was not removed or 
>> re-created. */
>> +        if (pci_get_pdev_by_domain(d, sbdf.seg, sbdf.bus, sbdf.devfn) != 
>> pdev)
>> +            if ( vpci is NOT the same )
>> +            {
>> +                rc = 0;
>> +                break;
>> +            }
>> +    }
>> +
>>        rangeset_destroy(mem);
>>        if ( !rc )
>>            modify_decoding(pdev, cmd, false);
>>
>> This one also wants process_pending_softirqs to run so it *might*
>> want pdev and vpci checks. But at the same time apply_map runs
>> at ( system_state < SYS_STATE_active ), so defer_map won't be
>> running yet, thus no vpci_process_pending is possible yet (in terms
>> it has something to do yet). So, I think we just need:
>>
>>           write_unlock(&pdev->domain->vpci_rwlock);
>>           process_pending_softirqs();
>>           write_lock(&pdev->domain->vpci_rwlock);
>>
>> and this should be enough
> Given the context apply_map is called from (dom0 specific init code),
> there's no need to check for the pdev to still exits, or whether vpci
> has been recreated, as it's not possible. Just add a comment to
> explicitly note that the context of the function is special, and thus
> there's no possibility of either the device or vpci going away.
Does it really need write_unlock/write_lock given the context?...
I think it doesn't as there is no chance defer_map is called, thus
process_pending_softirqs -> vpci_process_pending -> read_lock
is not possible
I'll just add a comment about that
> Thanks, Roger.
Thank you,
Oleksandr