Re: [PATCH v4 00/10] riscv: Allow userspace to directly access perf counters

[Atish Patra]

On Mon, Jul 3, 2023 at 5:46 AM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
>
> riscv used to allow direct access to cycle/time/instret counters,
> bypassing the perf framework, this patchset intends to allow the user to
> mmap any counter when accessed through perf. But we can't break the
> existing behaviour so we introduce a sysctl perf_user_access like arm64
> does, which defaults to the legacy mode described above.
>
> This version needs openSBI v1.3 *and* a kernel fix that went upstream lately
> (https://lore.kernel.org/lkml/20230616114831.3186980-1-maz@xxxxxxxxxx/T/).
>
> **Important**: In this version, the default mode is now user access, not
> the legacy so some applications will break.
>
Maybe providing a little more context to the issue will help
understanding why breaking those legacy
applications is necessary due to security reasons.

Here was the previous discussion around this[1].
Most of the legacy user applications using rdcycle should use rdtime
instead as they just want to record the time
elapsed. Allowing rdcycle/rdinstret to be read from user space can
lead to very deterministic attacks[2].

[1] https://groups.google.com/a/groups.riscv.org/g/sw-dev/c/REWcwYnzsKE?pli=1
[2] https://www.youtube.com/watch?v=3-c4C_L2PRQ&ab_channel=IEEESymposiumonSecurityandPrivacy

Any user application that really requires to read rdcycle directly can
use this new perf interface to do that without any latency.

> base-commit-tag: v6.4-rc6
>
> Changes in v4:
> - Fixed some nits in riscv_pmu_sbi.c thanks to Andrew
> - Fixed the documentation thanks to Andrew
> - Added RB from Andrew \o/
>
> Changes in v3:
> - patch 1 now contains the ref to the faulty commit (no Fixes tag as it is only a comment), as Andrew suggested
> - Removed RISCV_PMU_LEGACY_TIME from patch 3, as Andrew suggested
> - Rename RISCV_PMU_PDEV_NAME to "riscv-pmu-sbi", patch4 is just cosmetic now, as Andrew suggested
> - Removed a few useless (and wrong) comments, as Andrew suggested
> - Simplify arch_perf_update_userpage code, as Andrew suggested
> - Documentation now mentions that time CSR is *always* accessible, whatever the mode, as suggested by Andrew
> - Removed CYCLEH reference and add TODO for rv32 support, as suggested by Atish
> - Do not rename the pmu instance as Atish suggested
> - Set pmc_width only if rdpmc is enabled and CONFIG_RISCV_PMU is set and the event is a hw event
> - Move arch_perf_update_userpage https://lore.kernel.org/lkml/20230616114831.3186980-1-maz@xxxxxxxxxx/T/
> - **Switch to user mode access by default**
>
> Changes in v2:
> - Split into smaller patches, way better!
> - Add RB from Conor
> - Simplify the way we checked riscv architecture
> - Fix race mmap and other thread running on other cpus
> - Use hwc when available
> - Set all userspace access flags in event_init, too cumbersome to handle sysctl changes
> - Fix arch_perf_update_userpage for pmu other than riscv-pmu by renaming pmu driver
> - Fixed kernel test robot build error
> - Fixed documentation (Andrew and Bagas)
> - perf testsuite passes mmap tests in all 3 modes
>
> Alexandre Ghiti (10):
>   perf: Fix wrong comment about default event_idx
>   include: riscv: Fix wrong include guard in riscv_pmu.h
>   riscv: Make legacy counter enum match the HW numbering
>   drivers: perf: Rename riscv pmu sbi driver
>   riscv: Prepare for user-space perf event mmap support
>   drivers: perf: Implement perf event mmap support in the legacy backend
>   drivers: perf: Implement perf event mmap support in the SBI backend
>   Documentation: admin-guide: Add riscv sysctl_perf_user_access
>   tools: lib: perf: Implement riscv mmap support
>   perf: tests: Adapt mmap-basic.c for riscv
>
>  Documentation/admin-guide/sysctl/kernel.rst |  27 ++-
>  drivers/perf/riscv_pmu.c                    | 113 +++++++++++
>  drivers/perf/riscv_pmu_legacy.c             |  28 ++-
>  drivers/perf/riscv_pmu_sbi.c                | 196 +++++++++++++++++++-
>  include/linux/perf/riscv_pmu.h              |  12 +-
>  include/linux/perf_event.h                  |   3 +-
>  tools/lib/perf/mmap.c                       |  65 +++++++
>  tools/perf/tests/mmap-basic.c               |   4 +-
>  8 files changed, 428 insertions(+), 20 deletions(-)
>
> --
> 2.39.2
>


-- 
Regards,
Atish