On Fri, Aug 07, 2020 at 05:05:17PM -0600, Rob Herring wrote: > x86 and arm64 can both support direct access of event counters in > userspace. The access sequence is less than trivial and currently exists > in perf test code (tools/perf/arch/x86/tests/rdpmc.c) with copies in > projects such as PAPI and libpfm4. > > Patches to add arm64 userspace support are pending[1]. > > For this RFC, looking for a yes, seems like a good idea, or no, go away we > don't want this in libperf.
I'd like it lots better if you'd at least take an optimized version of this, also see this thread: https://lkml.kernel.org/r/[email protected] Also, I usually strip out all the multiplexing crud out (and use pinned counters), which saves a bunch. static inline u64 mmap_read_pinned(void *addr) { struct perf_event_mmap_page *pc = addr; u32 seq, idx, width = 0; u64 count; s64 pmc = 0; do { seq = pc->lock; barrier(); idx = pc->index; count = pc->offset; if (pc->cap_user_rdpmc && idx) { width = pc->pmc_width; pmc = rdpmc(idx - 1); } barrier(); } while (pc->lock != seq); if (idx) { pmc <<= 64 - width; pmc >>= 64 - width; /* shift right signed */ count += pmc; } return count; }
