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Subject[PATCH] perf: Introduce function to report unsupported syscall attribute flags
Date
Often a syscall fails with

Error: sys_perf_event_open() syscall returned with 95 (Operation not supported). /bin/dmesg may provide additional information.

but no additional information.

This patch adds some macro magic and a helper function to check and
report unsupported syscall attribute flags:

has_unsupported_flags(attr1, attr2);
has_unsupported_flag(attr, flag);
__has_unsupported_flags(const struct perf_event_attr *attr,
u64 notsup);

E.g. dmesg reports the following (checking for the exclude_guest
flag):

perf: unsupported attribute flags: 0000000000100000

or (checking precise modifier :ppp):

perf: unsupported attribute flags: 0000000000018000

Signed-off-by: Robert Richter <robert.richter@amd.com>
---
arch/x86/kernel/cpu/perf_event.c | 19 ++++++++++---------
arch/x86/kernel/cpu/perf_event_amd_ibs.c | 3 ++-
include/linux/perf_event.h | 23 +++++++++++++++++++++++
kernel/events/core.c | 2 +-
4 files changed, 36 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 915b876..9ae8676 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -336,7 +336,7 @@ int x86_setup_perfctr(struct perf_event *event)
return -EOPNOTSUPP;

/* BTS is currently only allowed for user-mode. */
- if (!attr->exclude_kernel)
+ if (has_unsupported_flag(attr, exclude_kernel))
return -EOPNOTSUPP;
}

@@ -389,8 +389,11 @@ int x86_pmu_hw_config(struct perf_event *event)
precise++;
}

- if (event->attr.precise_ip > precise)
+ if (event->attr.precise_ip > precise) {
+ has_unsupported_flag(&event->attr, precise_ip);
return -EOPNOTSUPP;
+ }
+
/*
* check that PEBS LBR correction does not conflict with
* whatever the user is asking with attr->branch_sample_type
@@ -398,13 +401,7 @@ int x86_pmu_hw_config(struct perf_event *event)
if (event->attr.precise_ip > 1) {
u64 *br_type = &event->attr.branch_sample_type;

- if (has_branch_stack(event)) {
- if (!precise_br_compat(event))
- return -EOPNOTSUPP;
-
- /* branch_sample_type is compatible */
-
- } else {
+ if (!has_branch_stack(event)) {
/*
* user did not specify branch_sample_type
*
@@ -419,7 +416,11 @@ int x86_pmu_hw_config(struct perf_event *event)

if (!event->attr.exclude_kernel)
*br_type |= PERF_SAMPLE_BRANCH_KERNEL;
+ } else if (!precise_br_compat(event)) {
+ has_unsupported_flag(&event->attr, precise_ip);
+ return -EOPNOTSUPP;
}
+ /* else: branch_sample_type is compatible */
}
}

diff --git a/arch/x86/kernel/cpu/perf_event_amd_ibs.c b/arch/x86/kernel/cpu/perf_event_amd_ibs.c
index 4af8275..fc145b0 100644
--- a/arch/x86/kernel/cpu/perf_event_amd_ibs.c
+++ b/arch/x86/kernel/cpu/perf_event_amd_ibs.c
@@ -186,6 +186,7 @@ static int perf_ibs_precise_event(struct perf_event *event, u64 *config)
case 2:
break;
default:
+ has_unsupported_flag(&event->attr, precise_ip);
return -EOPNOTSUPP;
}

@@ -243,7 +244,7 @@ static int perf_ibs_init(struct perf_event *event)
if (event->pmu != &perf_ibs->pmu)
return -ENOENT;

- if (perf_flags(&event->attr) & perf_flags(&ibs_notsupp))
+ if (has_unsupported_flags(&event->attr, &ibs_notsupp))
return -EINVAL;

if (config & ~perf_ibs->config_mask)
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index c36a04f..eb247a5 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -305,6 +305,14 @@ struct perf_event_attr {
};

#define perf_flags(attr) (*(&(attr)->read_format + 1))
+#define perf_flags_init(flag) ({ \
+ volatile __u64 flags = 0; \
+ struct perf_event_attr *attr; \
+ attr = (void *)((char *)&flags - \
+ (char *)&perf_flags((struct perf_event_attr *)(0))); \
+ if (!WARN_ON(&perf_flags(attr) != &flags)) \
+ attr->flag = ~0; \
+ flags;})

/*
* Ioctls that can be done on a perf event fd:
@@ -1208,6 +1216,21 @@ extern int perf_event_overflow(struct perf_event *event,
struct perf_sample_data *data,
struct pt_regs *regs);

+static inline bool __has_unsupported_flags(const struct perf_event_attr *attr,
+ u64 notsup)
+{
+ notsup &= perf_flags(attr);
+ if (notsup)
+ pr_warn("perf: unsupported attribute flags: %016llx\n", notsup);
+ return notsup != 0;
+}
+
+#define has_unsupported_flags(attr1, attr2) \
+ __has_unsupported_flags((attr1), perf_flags(attr2))
+/* flag is the name of the member in struct perf_event_attr: */
+#define has_unsupported_flag(attr, flag) \
+ __has_unsupported_flags((attr), perf_flags_init(flag))
+
static inline bool is_sampling_event(struct perf_event *event)
{
return event->attr.sample_period != 0;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 2ba8904..2667ef6 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6291,7 +6291,7 @@ static int perf_copy_attr(struct perf_event_attr __user *uattr,
if (ret)
return -EFAULT;

- if (attr->__reserved_1)
+ if (has_unsupported_flag(attr, __reserved_1))
return -EINVAL;

if (attr->sample_type & ~(PERF_SAMPLE_MAX-1))
--
1.7.8.6



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