Commit b2c04fc1 authored Jan 23, 2026 by Thomas Richter Committed by Heiko Carstens Jan 27, 2026

s390/perf: Disable register readout on sampling events



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>

parent 0d453ba0

arch/s390/kernel/perf_cpum_sf.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -841,7 +841,7 @@ 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)