[PATCH v3 0/2] KVM: arm64: PMU: Correct the handling of PMUSERENR_EL0

[Reiji Watanabe]

This series will fix bugs in KVM's handling of PMUSERENR_EL0.

With PMU access support from EL0 [1], the perf subsystem would
set CR and ER bits of PMUSERENR_EL0 as needed to allow EL0 to have
a direct access to PMU counters.  However, KVM appears to assume
that the register value is always zero for the host EL0, and has
the following two problems in handling the register.

[A] The host EL0 might lose the direct access to PMU counters, as
    KVM always clears PMUSERENR_EL0 before returning to userspace.

[B] With VHE, the guest EL0 access to PMU counters might be trapped
    to EL1 instead of to EL2 (even when PMUSERENR_EL0 for the guest
    indicates that the guest EL0 has an access to the counters).
    This is because, with VHE, KVM sets ER, CR, SW and EN bits of
    PMUSERENR_EL0 to 1 on vcpu_load() to ensure to trap PMU access
    from the guset EL0 to EL2, but those bits might be cleared by
    the perf subsystem after vcpu_load() (when PMU counters are
    programmed for the vPMU emulation).

Patch-1 will fix [A], and Patch-2 will fix [B] respectively.
The series is based on v6.3-rc6.

v3:
 - While vcpu_{put,load}() are manipulating PMUSERENR_EL0,
   disable IRQs to prevent a race condition between these
   processes and IPIs that updates PMUSERENR_EL0. [Mark]

v2: https://lore.kernel.org/all/20230408034759.2369068-1-reijiw@google.com/
 - Save the PMUSERENR_EL0 for the host in the sysreg array of
   kvm_host_data. [Marc]
 - Don't let armv8pmu_start() overwrite PMUSERENR if the vCPU
   is loaded, instead have KVM update the saved shadow register
   value for the host. [Marc, Mark]

v1: https://lore.kernel.org/all/20230329002136.2463442-1-reijiw@google.com/

[1] https://github.com/torvalds/linux/commit/83a7a4d643d33a8b74a42229346b7ed7139fcef9

Reiji Watanabe (2):
  KVM: arm64: PMU: Restore the host's PMUSERENR_EL0
  KVM: arm64: PMU: Don't overwrite PMUSERENR with vcpu loaded

 arch/arm64/include/asm/kvm_host.h       |  7 +++++
 arch/arm64/kernel/perf_event.c          | 21 ++++++++++++--
 arch/arm64/kvm/hyp/include/hyp/switch.h | 37 +++++++++++++++++++++++--
 arch/arm64/kvm/hyp/nvhe/Makefile        |  2 +-
 arch/arm64/kvm/pmu.c                    | 25 +++++++++++++++++
 include/linux/irqflags.h                |  4 +--
 6 files changed, 88 insertions(+), 8 deletions(-)


base-commit: 09a9639e56c01c7a00d6c0ca63f4c7c41abe075d
-- 
2.40.0.634.g4ca3ef3211-goog