Re: [PATCH 0/8] s390x/pci: Fixing s390 vfio-pci ISM support

[Pierre Morel <pmorel@linux.ibm.com>]

On 1/21/21 10:58 AM, Niklas Schnelle wrote:
> 
> 
> On 1/21/21 9:27 AM, Pierre Morel wrote:
>>
>>
>> On 1/20/21 9:29 PM, Matthew Rosato wrote:
>>> On 1/20/21 2:18 PM, Pierre Morel wrote:
>>>>
>>>>
>> ...snip...
>>
>>>
>>> So, I mean I can change the code to be more permissive in that way (allow any device that doesn't have MSI-X capability to at least attempt to use the region).  But the reality is that ISM specifically needs the region for successful pass through, so I don't see a reason to create a different bit for that vs just checking for the PFT in QEMU and using that value to decide whether or not region availability is a requirement for allowing the device to pass through.
>>
>>
>> There is no need for a new bit to know if a device support MIO or not, as I said before, there is already one in the CLP query PCI function response and it is already used in the kernel zPCI architecture.
>>
>>
>> It is not a big think to do and does not change the general architecture of the patch, only the detection of which device is impacted to make it generic instead of device dedicated.
>>
>> Regards,
>> Pierre
> 
> Just wanted to say that we've had a very similar discussion with
> Cornelia end of last year and came to the conclusion that explicitly
> matching the PFT is likely the safest bet:
> https://lkml.org/lkml/2020/12/22/479

What I see there is a discussion on the relation between relaxed access 
and MIO without explaining to Connie that we have in the kernel the 
possibility to know if a device support MIO or not independently of it 
supports the relaxed access.

The all point here is about taking decisions for the right reasons.

We have the possibility to take the decision based on functionalities 
and not on a specific PCI function.


If we keep the PFT check, and we can do this of course, but is it a good 
solution if it appears we have other PFT with the same functionalities?

Please note that this is a minor code change, keeping track of the MIO 
support just as we keep track of the PFT and check on this instead of on 
the PFT.

It does not modify the general architecture of the patch series neither 
in kernel nor in QEMU at all.


Pierre

-- 
Pierre Morel
IBM Lab Boeblingen