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The interaction between the exclude_{host,guest} flags,
exclude_{user,kernel,hv} flags and presence of VHE can result in
different exception levels being filtered by the ARMv8 PMU. As this
can be confusing let's document how they work on arm64.
Signed-off-by: Andrew Murray <andrew.murray@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
86 lines
2.6 KiB
Plaintext
86 lines
2.6 KiB
Plaintext
Perf Event Attributes
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=====================
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Author: Andrew Murray <andrew.murray@arm.com>
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Date: 2019-03-06
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exclude_user
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------------
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This attribute excludes userspace.
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Userspace always runs at EL0 and thus this attribute will exclude EL0.
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exclude_kernel
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--------------
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This attribute excludes the kernel.
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The kernel runs at EL2 with VHE and EL1 without. Guest kernels always run
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at EL1.
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For the host this attribute will exclude EL1 and additionally EL2 on a VHE
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system.
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For the guest this attribute will exclude EL1. Please note that EL2 is
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never counted within a guest.
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exclude_hv
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----------
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This attribute excludes the hypervisor.
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For a VHE host this attribute is ignored as we consider the host kernel to
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be the hypervisor.
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For a non-VHE host this attribute will exclude EL2 as we consider the
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hypervisor to be any code that runs at EL2 which is predominantly used for
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guest/host transitions.
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For the guest this attribute has no effect. Please note that EL2 is
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never counted within a guest.
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exclude_host / exclude_guest
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----------------------------
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These attributes exclude the KVM host and guest, respectively.
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The KVM host may run at EL0 (userspace), EL1 (non-VHE kernel) and EL2 (VHE
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kernel or non-VHE hypervisor).
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The KVM guest may run at EL0 (userspace) and EL1 (kernel).
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Due to the overlapping exception levels between host and guests we cannot
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exclusively rely on the PMU's hardware exception filtering - therefore we
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must enable/disable counting on the entry and exit to the guest. This is
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performed differently on VHE and non-VHE systems.
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For non-VHE systems we exclude EL2 for exclude_host - upon entering and
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exiting the guest we disable/enable the event as appropriate based on the
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exclude_host and exclude_guest attributes.
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For VHE systems we exclude EL1 for exclude_guest and exclude both EL0,EL2
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for exclude_host. Upon entering and exiting the guest we modify the event
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to include/exclude EL0 as appropriate based on the exclude_host and
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exclude_guest attributes.
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The statements above also apply when these attributes are used within a
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non-VHE guest however please note that EL2 is never counted within a guest.
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Accuracy
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--------
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On non-VHE hosts we enable/disable counters on the entry/exit of host/guest
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transition at EL2 - however there is a period of time between
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enabling/disabling the counters and entering/exiting the guest. We are
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able to eliminate counters counting host events on the boundaries of guest
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entry/exit when counting guest events by filtering out EL2 for
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exclude_host. However when using !exclude_hv there is a small blackout
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window at the guest entry/exit where host events are not captured.
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On VHE systems there are no blackout windows.
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