Re: [PATCH 2/2] iommu/amd: Remove performance counter pre-initialization test

["Suthikulpanit, Suravee" <suravee.suthikulpanit@amd.com>]

David / Joerg,

On 4/10/2021 5:03 PM, David Coe wrote:
> 
> The immediately obvious difference is the with the enormous count seen on mem_dte_mis on the older Ryzen 2400G. Will do some RTFM but anyone with comments and insight?
> 
> 841,689,151,202,939       amd_iommu_0/mem_dte_mis/              (33.44%)
> 
> Otherwise, all seems to running smoothly (especially for a distribution still in β). Bravo and many thanks all!

The initial hypothesis is that the issue happens only when users specify more number of events than
the available counters, which Perf will time-multiplex the events onto the counters.

Looking at the Perf and AMD IOMMU PMU multiplexing logic, it requires:
  1. Stop the counter (i.e. set CSOURCE to zero to stop counting)
  2. Save the counter value of the current event
  3. Reload the counter value of the new event (previously saved)
  4. Start the counter (i.e. set CSOURCE to count new events)

The problem here is that when the driver writes zero to CSOURCE register in step 1, this would enable power-gating,
which prevents access to the counter and result in writing/reading value in step 2 and 3.

I have found a system that reproduced this case (w/ unusually large number of count), and debug the issue further.
As a hack, I have tried skipping step 1, and it seems to eliminate this issue. However, this is logically incorrect,
and might result in inaccurate data depending on the events.

Here are the options:
1. Continue to look for workaround for this issue.
2. Find a way to disable event time-multiplexing (e.g. only limit the number of counters to 8)
    if power gating is enabled on the platform.
3. Back to the original logic where we had the pre-init check of the counter vlues, which is still the safest choice
    at the moment unless

Regards,
Suravee