[PATCH v2 18/18] perf: arm_spe: Disallow userspace profiling when arm_kernel_unmapped_at_el0()

[Will Deacon]

When running with the kernel unmapped whilst at EL0, the virtually-addressed
SPE buffer is also unmapped, which can lead to buffer faults if userspace
profiling is enabled.

This patch prohibits SPE profiling of userspace when
arm_kernel_unmapped_at_el0().

Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
---
 drivers/perf/arm_spe_pmu.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
index 8ce262fc2561..c028db8973a4 100644
--- a/drivers/perf/arm_spe_pmu.c
+++ b/drivers/perf/arm_spe_pmu.c
@@ -675,6 +675,13 @@ static int arm_spe_pmu_event_init(struct perf_event *event)
 		return -EOPNOTSUPP;
 
 	/*
+	 * If kernelspace is unmapped when running at EL0, then the SPE
+	 * buffer will fault and prematurely terminate the AUX session.
+	 */
+	if (arm64_kernel_unmapped_at_el0() && !attr->exclude_user)
+		dev_warn_once(&spe_pmu->pdev->dev, "unable to write to profiling buffer from EL0. Try passing \"kaiser=off\" on the kernel command line");
+
+	/*
 	 * Feedback-directed frequency throttling doesn't work when we
 	 * have a buffer of samples. We'd need to manually count the
 	 * samples in the buffer when it fills up and adjust the event
-- 
2.1.4