[Bug 201753] AMD-Vi: Unable to write to IOMMU perf counter

bugzilla-daemon
2021-03-25T21:52:52Z

--- Comment #33 from David Coe (david.coe@live.co.uk) ---
Good evening all!

Herewith a summary of our various results, including my latest for the Ryzen 7
4700U (Acer Swift 3).

20 msec wait with logged retrys

Ryzen Kernel Cold Warm

4700U 5.11.0-11 6 1
3500U 5.11.7 5 6
2500U 5.8.0-45 5
5.12.0 RC3 5 > 5
2400G 5.11.0-11 6 6
5.8.0-45 5 6
2200G ? >10

Two points are clear:

1. there are differences between cold and warm boot, mostly marginal but marked
and very consistent with my 4700U.

2. the choice of 5 as the maximum retry number is (sorry, Suravee) unfortunate.
Mostly, it guarantees that all our Ryzens just fail the IOMMU write test!

Paul's experiences with the 2200G are the exception and I can appreciate his
unease about just increasing the number of retrys. I'm sure he, Suravee, Alex
and the AMD engineers will find an elegant solution to the problem :-).

In the meantime, could you formally patch-the-patch, Suravee? I'll pass on the
above data to Alex Hung, our "cherry-picker" at Ubuntu.

Best regards

David