a9bf3130eb
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
|
|
=====================
|
|
|
|
Author: Andrew Murray <andrew.murray@arm.com>
|
|
Date: 2019-03-06
|
|
|
|
exclude_user
|
|
------------
|
|
|
|
This attribute excludes userspace.
|
|
|
|
Userspace always runs at EL0 and thus this attribute will exclude EL0.
|
|
|
|
|
|
exclude_kernel
|
|
--------------
|
|
|
|
This attribute excludes the kernel.
|
|
|
|
The kernel runs at EL2 with VHE and EL1 without. Guest kernels always run
|
|
at EL1.
|
|
|
|
For the host this attribute will exclude EL1 and additionally EL2 on a VHE
|
|
system.
|
|
|
|
For the guest this attribute will exclude EL1. Please note that EL2 is
|
|
never counted within a guest.
|
|
|
|
|
|
exclude_hv
|
|
----------
|
|
|
|
This attribute excludes the hypervisor.
|
|
|
|
For a VHE host this attribute is ignored as we consider the host kernel to
|
|
be the hypervisor.
|
|
|
|
For a non-VHE host this attribute will exclude EL2 as we consider the
|
|
hypervisor to be any code that runs at EL2 which is predominantly used for
|
|
guest/host transitions.
|
|
|
|
For the guest this attribute has no effect. Please note that EL2 is
|
|
never counted within a guest.
|
|
|
|
|
|
exclude_host / exclude_guest
|
|
----------------------------
|
|
|
|
These attributes exclude the KVM host and guest, respectively.
|
|
|
|
The KVM host may run at EL0 (userspace), EL1 (non-VHE kernel) and EL2 (VHE
|
|
kernel or non-VHE hypervisor).
|
|
|
|
The KVM guest may run at EL0 (userspace) and EL1 (kernel).
|
|
|
|
Due to the overlapping exception levels between host and guests we cannot
|
|
exclusively rely on the PMU's hardware exception filtering - therefore we
|
|
must enable/disable counting on the entry and exit to the guest. This is
|
|
performed differently on VHE and non-VHE systems.
|
|
|
|
For non-VHE systems we exclude EL2 for exclude_host - upon entering and
|
|
exiting the guest we disable/enable the event as appropriate based on the
|
|
exclude_host and exclude_guest attributes.
|
|
|
|
For VHE systems we exclude EL1 for exclude_guest and exclude both EL0,EL2
|
|
for exclude_host. Upon entering and exiting the guest we modify the event
|
|
to include/exclude EL0 as appropriate based on the exclude_host and
|
|
exclude_guest attributes.
|
|
|
|
The statements above also apply when these attributes are used within a
|
|
non-VHE guest however please note that EL2 is never counted within a guest.
|
|
|
|
|
|
Accuracy
|
|
--------
|
|
|
|
On non-VHE hosts we enable/disable counters on the entry/exit of host/guest
|
|
transition at EL2 - however there is a period of time between
|
|
enabling/disabling the counters and entering/exiting the guest. We are
|
|
able to eliminate counters counting host events on the boundaries of guest
|
|
entry/exit when counting guest events by filtering out EL2 for
|
|
exclude_host. However when using !exclude_hv there is a small blackout
|
|
window at the guest entry/exit where host events are not captured.
|
|
|
|
On VHE systems there are no blackout windows.
|