Re: [PATCH] iommu/amd: Fix event counter availability check

[Alexander Monakov <amonakov@ispras.ru>]

On Tue, 2 Jun 2020, Shuah Khan wrote:

> I changed the logic to read config to get max banks and counters
> before checking if counters are writable and tried writing to all.
> The result is the same and all of them aren't writable. However,
> when disable the writable check and assume they are, I can run
[snip]

This is similar to what I did. I also noticed that counters can
be successfully used with perf if the initial check is ignored.
I was considering sending a patch to remove the check and adjust
the event counting logic to use counters as read-only, but after
a bit more investigation I've noticed how late pci_enable_device
is done, and came up with this patch. It's a path of less resistance:
I'd expect maintainers to be more averse to removing the check
rather than fixing it so it works as intended (even though I think
the check should not be there in the first place).

However:

The ability to modify the counters is needed only for sampling the
events (getting an interrupt when a counter overflows). There's no
code to do that for these AMD IOMMU counters. A solution I would
prefer is to not write to those counters at all. It would simplify or
even remove a bunch of code. I can submit a corresponding patch if
there's general agreement this path is ok.

What do you think?

Alexander