Re: [patch 10/10] perf_event_open.2: 4.0 update rdpmc documentation

[Andy Lutomirski <luto@amacapital.net>]

On 04/16/2015 11:20 AM, Vince Weaver wrote:
>
> The rdpmc instruction allows reading performance counters directly
> from usersapce.  Prior to Linux 4.0 any process could use this
> instruction when a perf event was running, even if the process itself
> did not have any open.  The following changesets changed the default
> behavior so that only processes with active events can use rdpmc.
>
> Note this change broke the ABI.  Previously:
> 	/sys/bus/event_source/devices/cpu/rdpmc
> Set to "1" meant allow across whole system.
>
> After the change "2" means the whole system, and "1" means per-process.
>
> Probably a better change would have been to add "2" to mean per-process
> and make that the default setting.  Probably too late to fix that now.

Good point.  I wish you'd thought of that sooner :(

--Andy

>
> 	commit a66734297f78707ce39d756b656bfae861d53f62
> 	Author: Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>
>
> 	perf/x86: Add /sys/devices/cpu/rdpmc=2 to allow rdpmc for all tasks
>
> 	commit 7911d3f7af14a614617e38245fedf98a724e46a9
> 	Author: Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>
>
> 	perf/x86: Only allow rdpmc if a perf_event is mapped
>
> 	Signed-off-by: Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>
> 	Signed-off-by: Peter Zijlstra (Intel) <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
> 	Cc: Paul Mackerras <paulus-eUNUBHrolfbYtjvyW6yDsg@public.gmane.org>
> 	Cc: Arnaldo Carvalho de Melo <acme-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> 	Cc: Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
> 	Cc: Andrea Arcangeli <aarcange-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> 	Cc: Vince Weaver <vince-yfjdyHUqu3OsTnJN9+BGXg@public.gmane.org>
> 	Cc: "hillf.zj" <hillf.zj-gPhfCIXyaqCqndwCJWfcng@public.gmane.org>
> 	Cc: Valdis Kletnieks <Valdis.Kletnieks-PjAqaU27lzQ@public.gmane.org>
> 	Cc: Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
> 	Link: http://lkml.kernel.org/r/caac3c1c707dcca48ecbc35f4def21495856f479.1414190806.git.luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org
> 	Signed-off-by: Ingo Molnar <mingo-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
>
> Signed-off-by: Vince Weaver <vincent.weaver-e7X0jjDqjFGHXe+LvDLADg@public.gmane.org>
>
> diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2
> index 01ee579..c854d21 100644
> --- a/man2/perf_event_open.2
> +++ b/man2/perf_event_open.2
> @@ -2377,6 +2377,16 @@ Support for this can be detected with the
>   .I cap_usr_rdpmc
>   field in the mmap page; documentation on how
>   to calculate event values can be found in that section.
> +
> +Originally when rdpmc support was enabled, any process (not just ones
> +with an active perf event) could use the rdpmc instruction to access
> +the counters.
> +Starting with Linux 4.0
> +.\" 7911d3f7af14a614617e38245fedf98a724e46a9
> +rdpmc support is only enabled if an event is currently enabled
> +in a process' context.
> +To restore the old behavior, write the value 2 to
> +.IR /sys/devices/cpu/rdpmc .
>   .SS perf_event ioctl calls
>   .PP
>   Various ioctls act on
> @@ -2552,11 +2562,18 @@ field of
>   .I perf_event_attr
>   to indicate that you wish to use this PMU.
>   .TP
> -.IR /sys/bus/event_source/devices/*/rdpmc " (since Linux 3.4)"
> +.IR /sys/bus/event_source/devices/cpu/rdpmc " (since Linux 3.4)"
>   .\" commit 0c9d42ed4cee2aa1dfc3a260b741baae8615744f
>   If this file is 1, then direct user-space access to the
>   performance counter registers is allowed via the rdpmc instruction.
>   This can be disabled by echoing 0 to the file.
> +
> +As of Linux 4.0
> +.\" a66734297f78707ce39d756b656bfae861d53f62
> +.\" 7911d3f7af14a614617e38245fedf98a724e46a9
> +the behavior has changed, so that 1 now means only allow access
> +to processes with active perf events, with 2 indicating the old
> +allow-anyone-access behavior.
>   .TP
>   .IR /sys/bus/event_source/devices/*/format/ " (since Linux 3.4)"
>   .\" commit 641cc938815dfd09f8fa1ec72deb814f0938ac33
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