From a9bf3130ebfe0378df449a1b13997c47a58e661a Mon Sep 17 00:00:00 2001 From: Andrew Murray Date: Tue, 9 Apr 2019 20:22:17 +0100 Subject: [PATCH] arm64: docs: Document perf event attributes 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 Reviewed-by: Suzuki K Poulose Acked-by: Will Deacon Signed-off-by: Marc Zyngier --- Documentation/arm64/perf.txt | 85 ++++++++++++++++++++++++++++++++++++ 1 file changed, 85 insertions(+) create mode 100644 Documentation/arm64/perf.txt diff --git a/Documentation/arm64/perf.txt b/Documentation/arm64/perf.txt new file mode 100644 index 000000000000..0d6a7d87d49e --- /dev/null +++ b/Documentation/arm64/perf.txt @@ -0,0 +1,85 @@ +Perf Event Attributes +===================== + +Author: Andrew Murray +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.