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From: Alexander Monakov <amonakov@ispras.ru>
To: Shuah Khan <skhan@linuxfoundation.org>
Cc: iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] iommu/amd: Fix event counter availability check
Date: Wed, 3 Jun 2020 09:54:42 +0300 (MSK)
Message-ID: <alpine.LNX.2.20.13.2006030935570.3181@monopod.intra.ispras.ru> (raw)
In-Reply-To: <c0f9f676-eff8-572d-9174-4c22c6095a3d@linuxfoundation.org>

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
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Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-29 20:07 Alexander Monakov
2020-05-31  7:22 ` Alexander Monakov
2020-06-01  2:48   ` Paul Menzel
2021-02-21 13:44     ` Paul Menzel
2020-06-02 23:51   ` Shuah Khan
2020-06-03  6:54     ` Alexander Monakov [this message]
2021-02-26 21:44       ` Paul Menzel
2021-02-26 21:55         ` Shuah Khan
2020-06-01  7:37 ` Suravee Suthikulpanit
2020-06-01  9:01   ` Alexander Monakov
2020-06-01 15:10     ` Suravee Suthikulpanit
2020-06-01 16:09       ` Alexander Monakov
2020-06-15 20:48       ` Alexander Monakov
2020-06-16  9:35         ` Suravee Suthikulpanit
2020-06-30 19:22           ` Alexander Monakov
2020-09-17 17:55           ` Alexander Monakov
2021-02-21 13:49             ` Paul Menzel
2021-02-22 17:59               ` Suravee Suthikulpanit
2021-02-24 20:35                 ` Paul Menzel

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