Re: [PATCH v4] x86/msi: fix locking for SR-IOV devices

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.10.2024 21:44, Stewart Hildebrand wrote:
> On 8/28/24 06:36, Jan Beulich wrote:
>> On 27.08.2024 05:59, Stewart Hildebrand wrote:
>>> --- a/xen/drivers/passthrough/pci.c
>>> +++ b/xen/drivers/passthrough/pci.c
>>> @@ -341,6 +341,8 @@ static struct pci_dev *alloc_pdev(struct pci_seg *pseg, 
>>> u8 bus, u8 devfn)
>>>  
>>>      list_add(&pdev->alldevs_list, &pseg->alldevs_list);
>>>  
>>> +    INIT_LIST_HEAD(&pdev->physfn.vf_list);
>>
>> There is a certain risk with doing such uniformly when the field is part
>> of a union. Yes, little initialization has happened up to here, but I'm
>> still concerned. (One of the reasons I don't like the struct list_head
>> instances to be split, despite your legitimate point regarding naming.)
>> At the very least this wants moving yet earlier in the function, before
>> the new struct is passed anywhere else.
> 
> Understood. I personally have a slight preference for keeping the entry
> and head names distinct, so I'll plan to move the initialization
> earlier. However, I could easily be convinced to un-split the struct
> list_head instances if that's your preference. Let me know.

I indicated before that this would be my preference, without meaning to
insist on this folding.

>>> +                list_for_each_entry_safe(vf_pdev, tmp, 
>>> &pdev->physfn.vf_list,
>>> +                                         virtfn.entry)
>>> +                    ret = pci_remove_device(vf_pdev->sbdf.seg,
>>> +                                            vf_pdev->sbdf.bus,
>>> +                                            vf_pdev->sbdf.devfn) ?: ret;
>>
>> And if this fails, the VF will still remain orphaned. I think in the
>> model I had suggested no such risk would exist.
>>
>> Misra also isn't going to like the recursion here.
> 
> With the ASSERTs being addressed directly, there's no need to remove
> the VFs right away with the PF.
> 
> BTW, I don't think refusing a removal "request" would be a good idea.
> Dom0 isn't really requesting the device to be removed. Dom0 has already
> removed the device (e.g. in response to hot-unplug or SR-IOV disable),
> and is merely informing Xen of the removal.

Just to clarify: I don't mean returning an error here to indicate
"refusal". As you say, this is merely a notification. Yet I think it's
still legitimate to pass back an error. Whether the Dom0 kernel can
do anything useful with that error is a separate question.

> So during PF removal, I'll plan (for v5) to unlink the the VFs from the
> PF, and continue to rely on dom0 to inform Xen of PF and VF removal
> individually. By unlink, I mean set vf_pdev->virtfn.pf_pdev = NULL and
> remove the VFs from the list. Probably also set vf_pdev->broken = true.

As to the latter - yes. For the rest I guess I need to see the new code.

Jan