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

From: Pierre Morel
Subject: Re: [PATCH 0/8] s390x/pci: Fixing s390 vfio-pci ISM support
Date: Thu, 21 Jan 2021 09:27:12 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0

On 1/20/21 9:29 PM, Matthew Rosato wrote:
On 1/20/21 2:18 PM, Pierre Morel wrote:


So we have:
devices supporting MIO and MSIX
devices not supporting MIO nor MSIX
devices not supporting the use of PCISTG to emulate PCISTB

The first two are two different things indicated by two different entries in the clp query PCI function response.

The last one, we do not have an indicator as if the relaxed alignment and length is set, PCISTB can not be emulated with PCISTG

hum sorry, it seems I rewrote my sentence until it was wrong wrong!
I wanted to say we DO HAVE an indicator with the relaxed bit...

What I mean with this is that considering the proposed implementation and considering:

0 0 1  -> must use the new region (ISM)
1 1 0  -> must use the standard VFIO region (MLX)

we can discuss other 6 possibilities

0 0 0 -> must use the new region
0 1 0 -> NOOP
0 1 1 -> NOOP
1 0 0 -> can use any region
1 0 1 -> can use any region
1 1 1 -> NOOP

In my opinion the test for using one region or another should be done on these indicator instead of using the PFT. > This may offer us more compatibility with other hardware we may not be
aware of as today.

This gets a little shaky, and goes both ways -- Using your list, a device that supports MIO, does not have MSI-X capability and doesn't support relaxed alignment (1 0 0 from above) can use any region -- but that may not always be true.  What if "other hardware we may not be aware of as today" includes future hardware that ONLY supports the MIO instruction set?  Then that device really can't use this region either.

Right, but there is no bit in the CLP response for this case.
Until there is one, the system is supposed to handle legacy instructions

But forgetting that possibility...  I think we can really simplify the above matrix down to a statement of "if device doesn't support MSI-X but DOES support non-MIO instructions, it can use the region."  I believe the latter half of that statement is implicit in the architecture today, so it's really then "if device doesn't support MSI-X, it can use the region".  There's just the caveat of, if the device is ISM, it changes from 'can use the region' to 'must use the region'.

There can surely be simplifications.

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.


Pierre Morel
IBM Lab Boeblingen

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