Re: [PATCH] perf_event_open.2: 4.9 PERF_EVENT_IOC_SET_DRV_CONFIGS support

On 12 August 2016 at 11:57, Vince Weaver [off-list ref] wrote:

On Thu, 11 Aug 2016, Mathieu Poirier wrote:

quoted

+.TP
+.BR PERF_EVENT_IOC_SET_DRV_CONFIGS " (since Linux 4.9)"
+This makes it possible to pass configuration parameters to PMU drivers.
+Configuration parameters are specific to each drivers.
+
+The argument is a pointer to the desired configuration parameters.

So I gave up and read the code and I'm guessing this is what you really
mean:

.TP
.BR PERF_EVENT_IOC_SET_DRV_CONFIGS " (since Linux 4.9)"
Pass custom configuration paramaters to a PMU driver.

The argument is a pointer to a NUL-termiated string of up to
PAGE_SIZE in length.
The string contains a list of comma-separated configuration options
that will be parsed by the kernel.
The kernel handles both singleton values as well as name/value pairs
that are indicated with the '=' character.
The size of the strings is limited internally to PERF_DRV_CONFIG_MAX
(which is not visible to userspace).

Yes this is a very good description but in sharp contrast with what is
currently done for the ioctl() descriptors in this page.  I shied away
from writing that much based on how slim the current descriptions
were.

some additional questions: can this ioctl be run at any time or should it
only be run while the event is quiet?  Does the change in options take
place immediately?

That is up to PMU drivers to decide - for CoreSight it is set only
once when trace sessions are created.  Any changes from thereon will
be ignored.

Thanks for the review.
Mathieu

Vince

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