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

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Date:   Wed, 3 Jun 2020 09:54:42 +0300 (MSK)
From:   Alexander Monakov <amonakov@...ras.ru>
To:     Shuah Khan <skhan@...uxfoundation.org>
cc:     linux-kernel@...r.kernel.org, Joerg Roedel <joro@...tes.org>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH] iommu/amd: Fix event counter availability check

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