[PATCH 6.18 065/752] s390/perf: Disable register readout on sampling events

Sasha Levin
2026-02-28T17:49:03Z

From: Thomas Richter <tmricht@linux.ibm.com>

[ Upstream commit b2c04fc1239062b39ddfdd8731ee1a10810dfb74 ]

Running commands
# ./perf record -IR0,R1 -a sleep 1
extracts and displays register value of general purpose register r1 and r0.
However the value displayed of any register is random and does not
reflect the register value recorded at the time of the sample interrupt.

The sampling device driver on s390 creates a very large buffer
for the hardware to store the samples. Only when that large buffer
gets full an interrupt is generated and many hundreds of sample
entries are processed and copied to the kernel ring buffer and
eventually get copied to the perf tool. It is during the copy
to the kernel ring buffer that each sample is processed (on s390)
and at that time the register values are extracted.
This is not the original goal, the register values should be read
when the samples are created not when the samples are copied to the
kernel ring buffer.

Prevent this event from being installed in the first place and
return -EOPNOTSUPP. This is already the case for PERF_SAMPLE_REGS_USER.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Jan Polensky <japo@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/s390/kernel/perf_cpum_sf.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/kernel/perf_cpum_sf.c b/arch/s390/kernel/perf_cpum_sf.c
index f432869f89213..9af7228d2d792 100644
--- a/arch/s390/kernel/perf_cpum_sf.c
+++ b/arch/s390/kernel/perf_cpum_sf.c
@@ -842,7 +842,7 @@ static bool is_callchain_event(struct perf_event *event)
u64 sample_type = event->attr.sample_type;

return sample_type & (PERF_SAMPLE_CALLCHAIN | PERF_SAMPLE_REGS_USER |
- PERF_SAMPLE_STACK_USER);
+ PERF_SAMPLE_REGS_INTR | PERF_SAMPLE_STACK_USER);
}

static int cpumsf_pmu_event_init(struct perf_event *event)
--
2.51.0