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KVM mediated PMU support for 6.20 Add support for mediated PMUs, where KVM gives the guest full ownership of PMU hardware (contexted switched around the fastpath run loop) and allows direct access to data MSRs and PMCs (restricted by the vPMU model), but intercepts access to control registers, e.g. to enforce event filtering and to prevent the guest from profiling sensitive host state. To keep overall complexity reasonable, mediated PMU usage is all or nothing for a given instance of KVM (controlled via module param). The Mediated PMU is disabled default, partly to maintain backwards compatilibity for existing setup, partly because there are tradeoffs when running with a mediated PMU that may be non-starters for some use cases, e.g. the host loses the ability to profile guests with mediated PMUs, the fastpath run loop is also a blind spot, entry/exit transitions are more expensive, etc. Versus the emulated PMU, where KVM is "just another perf user", the mediated PMU delivers more accurate profiling and monitoring (no risk of contention and thus dropped events), with significantly less overhead (fewer exits and faster emulation/programming of event selectors) E.g. when running Specint-2017 on a single-socket Sapphire Rapids with 56 cores and no-SMT, and using perf from within the guest: Perf command: a. basic-sampling: perf record -F 1000 -e 6-instructions -a --overwrite b. multiplex-sampling: perf record -F 1000 -e 10-instructions -a --overwrite Guest performance overhead: --------------------------------------------------------------------------- | Test case | emulated vPMU | all passthrough | passthrough with | | | | | event filters | --------------------------------------------------------------------------- | basic-sampling | 33.62% | 4.24% | 6.21% | --------------------------------------------------------------------------- | multiplex-sampling | 79.32% | 7.34% | 10.45% | --------------------------------------------------------------------------- |
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