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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.05.2021 16:38, Boris Ostrovsky wrote:
> 
> 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".

The common scanning logic is to look at slot 0 first. If that's populated,
other slots get looked at only if slot 0 has the multi-function bit set.
If slot 0 is not populated, nothing is known about the other slots, and
hence they need to be scanned.

Jan