Re: [PATCH v2 1/2] xen-pciback: redo VF placement in the virtual topology

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 5/20/21 3:43 AM, Jan Beulich wrote:
> On 20.05.2021 02:36, Boris Ostrovsky wrote:
>> On 5/18/21 12:13 PM, Jan Beulich wrote:
>>>  
>>> @@ -95,22 +95,25 @@ static int __xen_pcibk_add_pci_dev(struc
>>>  
>>>     /*
>>>      * Keep multi-function devices together on the virtual PCI bus, except
>>> -    * virtual functions.
>>> +    * that we want to keep virtual functions at func 0 on their own. They
>>> +    * aren't multi-function devices and hence their presence at func 0
>>> +    * may cause guests to not scan the other functions.
>>
>> So your reading of the original commit is that whatever the issue it was, 
>> only function zero was causing the problem? In other words, you are not 
>> concerned that pci_scan_slot() may now look at function 1 and skip all 
>> higher-numbered function (assuming the problem is still there)?
> I'm not sure I understand the question: Whether to look at higher numbered
> slots is a function of slot 0's multi-function bit alone, aiui. IOW if
> slot 1 is being looked at in the first place, slots 2-7 should also be
> looked at.


Wasn't the original patch describing a problem strictly as one for 
single-function devices, so the multi-function bit is not set? I.e. if all VFs 
(which are single-function devices) are placed in the same slot then 
pci_scan_slot() would only look at function 0 and ignore anything 
higher-numbered.


My question is whether it would "only look at function 0 and ignore anything 
higher-numbered" or "only look at the lowest-numbered function and ignore 
anything higher-numbered".


-boris