linux-kernel - Re: [PATCH v3 13/13] perf/x86: add syfs entry to disable HT bug workaround

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Date:	Tue, 18 Nov 2014 01:31:38 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Borislav Petkov <bp@...en8.de>
cc:	Stephane Eranian <eranian@...gle.com>, x86-ml <x86@...nel.org>,
	linux-kernel@...r.kernel.org, peterz@...radead.org, mingo@...e.hu,
	ak@...ux.intel.com, jolsa@...hat.com, kan.liang@...el.com,
	maria.n.dimakopoulou@...il.com
Subject: Re: [PATCH v3 13/13] perf/x86: add syfs entry to disable HT bug
 workaround

On Tue, 18 Nov 2014, Borislav Petkov wrote:
> On Tue, Nov 18, 2014 at 12:38:14AM +0100, Thomas Gleixner wrote:
> > Well a bitmask is a pretty indescriptive item as well. Putting my user
> > hat on: Where is the documentation for the bits?
> 
> $ cat /sys/devices/system/cpu/bugs
> 0xXXXXXX - currently enabled workarounds are the set bits.
> bit 0: workaround for bug#blabla
> bit 1: workaround for bug#1
> bit 2: workaround for bug#2; remember to do <bla> before disabling workaround
> ...
> bits n-63 are reserved, cannot be set and RAZ.

You sure that 64 are enough?

You need to create stable but numbers, i.e. each bug gets a fixed but
number whethr it affects the machine or not. Otherwise you will drive
admins completely nuts.

> This will be issued when user cats the sysfs file.

That might work as well, though you want that to be:

  /sys/devices/system/cpu/bugs/

  /sys/devices/system/cpu/bugs/status

  /sys/devices/system/cpu/bugs/enable_workaround

  /sys/devices/system/cpu/bugs/disable_workaround

  The latter two take a bit number rather than a magic mask.

So while it looks less effort to implement and extend in the first
place I think, that a bit of infrastructure work will make the
explicit scheme I proposed before a no brainer to maintain and extend,
but I cannot judge what's more intuitive to use.

Thanks,

	tglx



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