linux-kernel - Re: [PATCH V6 4/6] perf, x86: handle multiple records in PEBS buffer

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20150417081100.GZ24151@twins.programming.kicks-ass.net>
Date:	Fri, 17 Apr 2015 10:11:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kan Liang <kan.liang@...el.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, acme@...radead.org,
	eranian@...gle.com, andi@...stfloor.org
Subject: Re: [PATCH V6 4/6] perf, x86: handle multiple records in PEBS buffer

On Thu, Apr 16, 2015 at 02:53:42PM +0200, Peter Zijlstra wrote:
> When the PEBS interrupt threshold is larger than one record and the
> machine supports multiple PEBS events, the records of these events are
> mixed up and we need to demultiplex them.
> 
> Demuxing the records is hard because the hardware is deficient. The
> hardware has two issues that, when combined, create impossible scenarios
> to demux.
> 
> The first issue is that the 'status' field of the PEBS record is a copy
> of the GLOBAL_STATUS MSR at PEBS assist time. To see why this is a
> problem let us first describe the regular PEBS cycle:
> 
>  A) the CTRn value reaches 0:
>     - the corresponding bit in GLOBAL_STATUS gets set
>     - we start arming the hardware assist
> 
>     < some unspecified amount of time later --
>         this could cover multiple events of interest >
> 
>  B) the hardware assist is armed, any next event will trigger it
> 
>  C) a matching event happens:
>     - the hardware assist triggers and generates a PEBS record
>       this includes a copy of GLOBAL_STATUS at this moment
>     - if we auto-reload we (re)set CTRn

Is this actually true? Do we reload here or on A ?

>     - we clear the relevant bit in GLOBAL_STATUS
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/