drivers: perf: Implement perf event mmap support in the SBI backend

We used to unconditionnally expose the cycle and instret csrs to
userspace, which gives rise to security concerns.

So now we only allow access to hw counters from userspace through the perf
framework which will handle context switches, per-task events...etc. A
sysctl allows to revert the behaviour to the legacy mode so that userspace
applications which are not ready for this change do not break.

But the default value is to allow userspace only through perf: this will
break userspace applications which rely on direct access to rdcycle.
This choice was made for security reasons [1][2]: most of the applications
which use rdcycle can instead use rdtime to count the elapsed time.

[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

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/drivers/perf/riscv_pmu.c b/drivers/perf/riscv_pmu.c
index 432ad2e80ce3..80c052e93f9e 100644
--- a/drivers/perf/riscv_pmu.c
+++ b/drivers/perf/riscv_pmu.c
@@ -38,7 +38,15 @@ void arch_perf_update_userpage(struct perf_event *event,
 	userpg->cap_user_time_short = 0;
 	userpg->cap_user_rdpmc = riscv_perf_user_access(event);
 
-	userpg->pmc_width = 64;
+#ifdef CONFIG_RISCV_PMU
+	/*
+	 * The counters are 64-bit but the priv spec doesn't mandate all the
+	 * bits to be implemented: that's why, counter width can vary based on
+	 * the cpu vendor.
+	 */
+	if (userpg->cap_user_rdpmc)
+		userpg->pmc_width = to_riscv_pmu(event->pmu)->ctr_get_width(event->hw.idx) + 1;
+#endif
 
 	do {
 		rd = sched_clock_read_begin(&seq);