rdcycle from userland with RISCV_PMU_SBI=y

[Aurelien Jarno]

Hi all,

Mathieu Malaterre reported an issue in Debian with the highway package,
which uses the rdcycle pseudo instruction, and started to fail recently.

We tracked down an issue to the "perf platform driver based on SBI PMU
extension" recently added in Linux 5.18 [1]. On a board which runs an
SBI with the PMU extension, this causes the rdcycle pseudo instruction
to generate a SIGILL. This is because the driver explicitly set
CSR_SCOUNTEREN to 0x02, giving access from userland to rdtime but not
rdcycle.

I wonder if this change is intentional, and if there is a reason to
forbid using the rdcycle pseudo instruction from userland. If it is the
case, this should probably be changed so that the behaviour does not
differ from board to board depending on the available PMU extension.

Aurelien

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e9991434596f5373dfd75857b445eb92a9253c56
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/perf/riscv_pmu_sbi.c?id=2880e1a175b9f31798f9d9482ee49187f61b5539#n649

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien at aurel32.net                 http://www.aurel32.net