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The current code is incredibly resilient to updates to the spec and has worked quite well so far. However, recent implementations expose a weakness in that this is rather slow. A large part of it is written in assembly, making it opaque to the compiler for optimisations. The future proofness requires reading registers that are effectively `volatile`, making it even harder for the compiler, as well as adding lots of implicit barriers, making it hard for the microarchitecutre to optimise as well. We can make a few assumptions, checked by a few well placed asserts, and remove a lot of this burden. For a start, at the moment there are 4 group 0 counters with static assignments. Contexting them is a trivial affair that doesn't need a loop. Similarly, there can only be up to 16 group 1 counters. Contexting them is a bit harder, but we can do with a single branch with a falling through switch. If/when both of these change, we have a pair of asserts and the feature detection mechanism to guard us against pretending that we support something we don't. We can drop contexting of the offset registers. They are fully accessible by EL2 and as such are its responsibility to preserve on powerdown. Another small thing we can do, is pass the core_pos into the hook. The caller already knows which core we're running on, we don't need to call this non-trivial function again. Finally, knowing this, we don't really need the auxiliary AMUs to be described by the device tree. Linux doesn't care at the moment, and any information we need for EL3 can be neatly placed in a simple array. All of this, combined with lifting the actual saving out of assembly, reduces the instructions to save the context from 180 to 40, including a lot fewer branches. The code is also much shorter and easier to read. Also propagate to aarch32 so that the two don't diverge too much. Change-Id: Ib62e6e9ba5be7fb9fb8965c8eee148d5598a5361 Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
28 lines
1 KiB
ReStructuredText
28 lines
1 KiB
ReStructuredText
Activity Monitors
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=================
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FEAT_AMUv1 of the Armv8-A architecture introduces the Activity Monitors
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extension. This extension describes the architecture for the Activity Monitor
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Unit (|AMU|), an optional non-invasive component for monitoring core events
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through a set of 64-bit counters.
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When the ``ENABLE_FEAT_AMU=1`` build option is provided, Trusted Firmware-A
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sets up the |AMU| prior to its exit from EL3, and will save and restore
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architected |AMU| counters as necessary upon suspend and resume.
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.. _Activity Monitor Auxiliary Counters:
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Auxiliary counters
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------------------
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FEAT_AMUv1 describes a set of implementation-defined auxiliary counters (also
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known as group 1 counters), controlled by the ``ENABLE_AMU_AUXILIARY_COUNTERS``
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build option.
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As a security precaution, Trusted Firmware-A does not enable these by default.
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Instead, platforms must configure their auxiliary counters through the
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``plat_amu_aux_enables`` platform hook.
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--------------
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*Copyright (c) 2021-2025, Arm Limited. All rights reserved.*
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