Re: [RFC 2/2] perf: Marker software event and ioctl

[Arnaldo Carvalho de Melo <acme-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

Em Fri, Sep 12, 2014 at 01:57:52PM +0100, Pawel Moll escreveu:
> On Fri, 2014-09-12 at 13:43 +0100, Christopher Covington wrote:
> > Just to ask the dumb questions in case the answers I've come up with are
> > wrong: What is PAGE_SIZE on an arm64 kernel? 
 
> It's either 4 or 64k, depending on CONFIG_ARM64_64K_PAGES.
 
> > How does userspace know?

> #include <unistd.h>
> #include <stdio.h>
 
> int main(void)
> {
> 	printf("%ld\n", sysconf(_SC_PAGESIZE));
> 	return 0;
> }
 
> Now a word of explanation. The PAGE_SIZE limitation was shamelessly
> stolen from perf_event_set_filter() (so PERF_EVENT_IOC_SET_FILTER) as an
> attempt to address a problem of passing a zero-terminated string from
> userspace. Simply speaking - there must be some limitation, and a page
> size seem as good as any other. I have strong doubts about this myself,
> so all alternative ideas are more than welcome.
 
> As I mentioned in the cover letter, maybe this simply shouldn't be a
> string? I made it like this to mimic trace_marker, but maybe an integer
> value + some kind of a dictionary in userspace is a better approach? I
> belive that ftrace's maker is taking a string, because it's: 1. natural
> for its interface and 2. anyone (sort of) can write to it, so it's hard
> to assume anything. In this case the user "owns" the perf data, so he
> could handle int<->whatever-else relation table...

Perhaps both? I.e. an u64 followed from a string, if the u64 is zero,
then there is a string right after it?

- Arnaldo