The RISC-V SBI PMU specification defines several standard hardware and
cache events. Currently, all of these events are exposed to userspace,
even when not actually implemented. They appear in the `perf list`
output, and commands like `perf stat` try to use them.

This is more than just a cosmetic issue, because the PMU driver's .add
function fails for these events, which causes pmu_groups_sched_in() to
prematurely stop scheduling in other (possibly valid) hardware events.

Add logic to check which events are supported by the hardware (i.e. can
be mapped to some counter), so only usable events are reported to
userspace. Since the kernel does not know the mapping between events and
possible counters, this check must happen during boot, when no counters
are in use. Make the check asynchronous to minimize impact on boot time.

Fixes: e9991434 ("RISC-V: Add perf platform driver based on SBI PMU extension")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Tested-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20240628-misc_perf_fixes-v4-3-e01cfddcf035@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>