Re: [PATCH v3 bpf-next 1/4] tracing/probe: Add PERF_EVENT_IOC_QUERY_PROBE ioctl

[Song Liu]

Hi Peter, 

> On Aug 22, 2019, at 12:47 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> On Wed, Aug 21, 2019 at 06:43:49PM +0000, Yonghong Song wrote:
>> On 8/21/19 11:31 AM, Peter Zijlstra wrote:
> 
>>> So extending PERF_RECORD_LOST doesn't work. But PERF_FORMAT_LOST might
>>> still work fine; but you get to implement it for all software events.
>> 
>> Could you give more specifics about PERF_FORMAT_LOST? Googling 
>> "PERF_FORMAT_LOST" only yields two emails which we are discussing here :-(
> 
> Look at what the other PERF_FORMAT_ flags do? Basically it is adding a
> field to the read(2) output.

Do we need to implement PERF_FORMAT_LOST for all software events? If user
space asks for PERF_FORMAT_LOST for events that do not support it, can we
just fail sys_perf_event_open()?

Thanks,
Song