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commite10de31428upstream. On certain AMD platforms, when the IOMMU performance counter source (csource) field is zero, power-gating for the counter is enabled, which prevents write access and returns zero for read access. This can cause invalid perf result especially when event multiplexing is needed (i.e. more number of events than available counters) since the current logic keeps track of the previously read counter value, and subsequently re-program the counter to continue counting the event. With power-gating enabled, we cannot gurantee successful re-programming of the counter. Workaround this issue by : 1. Modifying the ordering of setting/reading counters and enabing/ disabling csources to only access the counter when the csource is set to non-zero. 2. Since AMD IOMMU PMU does not support interrupt mode, the logic can be simplified to always start counting with value zero, and accumulate the counter value when stopping without the need to keep track and reprogram the counter with the previously read counter value. This has been tested on systems with and without power-gating. Fixes:994d6608ef("iommu/amd: Remove performance counter pre-initialization test") Suggested-by: Alexander Monakov <amonakov@ispras.ru> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lkml.kernel.org/r/20210504065236.4415-1-suravee.suthikulpanit@amd.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.