The following commit has been merged into the perf/core branch of tip:
Commit-ID: 88c7bcad71c83f52f24108dedcecae0d18dbc627
Gitweb: https://git.kernel.org/tip/88c7bcad71c83f52f24108dedcecae0d18dbc627
Author: Ravi Bangoria <ravi.bangoria@amd.com>
AuthorDate: Wed, 15 Jan 2025 05:44:31
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 03 Feb 2025 11:46:04 +01:00
perf/amd/ibs: Remove pointless sample period check
Valid perf event sample period value for IBS PMUs (Fetch and Op both)
is limited to multiple of 0x10. perf_ibs_init() has this check:
if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
return -EINVAL;
But it's broken since hwc->sample_period will always be 0 when
event->attr.sample_freq is 0 (irrespective of event->attr.freq value.)
One option to fix this is to change the condition:
- if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
+ if (!event->attr.freq && hwc->sample_period & 0x0f)
However, that will break all userspace tools which have been using IBS
event with sample_period not multiple of 0x10.
Another option is to remove the condition altogether and mask lower
nibble _silently_, same as what current code is inadvertently doing.
I'm preferring this approach as it keeps the existing behavior.
Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/r/20250115054438.1021-3-ravi.bangoria@amd.com
---
arch/x86/events/amd/ibs.c | 9 ++-------
1 file changed, 2 insertions(+), 7 deletions(-)
diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
index 4ca8006..bd8919e 100644
--- a/arch/x86/events/amd/ibs.c
+++ b/arch/x86/events/amd/ibs.c
@@ -307,13 +307,8 @@ static int perf_ibs_init(struct perf_event *event)
if (config & perf_ibs->cnt_mask)
/* raw max_cnt may not be set */
return -EINVAL;
- if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
- /*
- * lower 4 bits can not be set in ibs max cnt,
- * but allowing it in case we adjust the
- * sample period to set a frequency.
- */
- return -EINVAL;
+
+ /* Silently mask off lower nibble. IBS hw mandates it. */
hwc->sample_period &= ~0x0FULL;
if (!hwc->sample_period)
hwc->sample_period = 0x10;