linux-kernel - Re: [patch 10/11] perf_event_open.2: 4.0 update rdpmc documentation

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Date:	Wed, 14 Oct 2015 08:46:48 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Vince Weaver <vincent.weaver@...ne.edu>
CC:	mtk.manpages@...il.com, linux-man@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Stephane Eranian <eranian@...gle.com>,
	Andy Lutomirski <luto@...capital.net>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"hillf.zj" <hillf.zj@...baba-inc.com>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 10/11] perf_event_open.2: 4.0 update rdpmc documentation

On 07/23/2015 07:16 PM, 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.


Thanks, Vince. Applied.

Cheers,

Michael


> 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.
> 
> 	commit a66734297f78707ce39d756b656bfae861d53f62
> 	Author: Andy Lutomirski <luto@...capital.net>
> 
> 	perf/x86: Add /sys/devices/cpu/rdpmc=2 to allow rdpmc for all tasks
> 
> 	commit 7911d3f7af14a614617e38245fedf98a724e46a9
> 	Author: Andy Lutomirski <luto@...capital.net>
> 
> 	perf/x86: Only allow rdpmc if a perf_event is mapped
> 
> 	Signed-off-by: Andy Lutomirski <luto@...capital.net>
> 	Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> 	Cc: Paul Mackerras <paulus@...ba.org>
> 	Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
> 	Cc: Kees Cook <keescook@...omium.org>
> 	Cc: Andrea Arcangeli <aarcange@...hat.com>
> 	Cc: Vince Weaver <vince@...ter.net>
> 	Cc: "hillf.zj" <hillf.zj@...baba-inc.com>
> 	Cc: Valdis Kletnieks <Valdis.Kletnieks@...edu>
> 	Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> 	Link: http://lkml.kernel.org/r/caac3c1c707dcca48ecbc35f4def21495856f479.1414190806.git.luto@amacapital.net
> 	Signed-off-by: Ingo Molnar <mingo@...nel.org>
> 
> Signed-off-by: Vince Weaver <vincent.weaver@...ne.edu>
> 
> diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2
> index 8ef9ec6..b3cd93f 100644
> --- a/man2/perf_event_open.2
> +++ b/man2/perf_event_open.2
> @@ -2595,6 +2595,16 @@ 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 allowed 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
> @@ -2783,11 +2793,18 @@ 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
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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