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 17:37:26 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Thomas Gleixner <tglx@...utronix.de>, x86-ml <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Jiri Olsa <jolsa@...hat.com>,
	"Liang, Kan" <kan.liang@...el.com>,
	Maria Dimakopoulou <maria.n.dimakopoulou@...il.com>
Subject: Re: [PATCH v3 13/13] perf/x86: add syfs entry to disable HT bug workaround

On Tue, Nov 18, 2014 at 4:43 PM, Borislav Petkov <bp@...en8.de> wrote:
> On Tue, Nov 18, 2014 at 04:29:59PM +0100, Stephane Eranian wrote:
>> I am trying to get a better understanding of this scheme.
>>
>> status:
>>   - a summary of what is enabled/disabled?
>>   - With description (as suggested by Boris)?
>>   - File is readonly
>>   - is that printing a variable length bitmask?
>
> Should be the easiest. Maybe extend/add to the X86_BUG() functionality
> in arch/x86/include/asm/cpufeature.h which already deals with CPU bugs.
>
Does it need to bit a bitmask, as opposed to just a bug number (which could
be implemented as a bitmask)?

>> enable_workaround:
>>    - provide the bit number (of the workaround) to enable the workaround
>
> Right, writing the bit number could be part of the description message
> above - just so that users know how to control the interface.
>
writing the bug number....

>>    - File is write-only
>>
>> disable_workaround:
>>    - provide the bit number (of the workaround) to disable the workaround
>>    - File is write-only
>>
>> The split enable/disable is to avoid the read-modify-write issue.
>>
>> Am I getting this right?
>>
>> I understand the value of this proposition. But, I feel,  it is beyond the scope
>> of the patch series to workaround the PMU bug. Initially, we had
>> talked about not
>> even providing the sysfs file. Now, the series adjusts the workaround
>> on boot. The series is restructured so that the sysfs patch is the last
>> one and is totally optional. I think we should implement the proposed scheme
>> but we should not delay the review and merge of the rest of the patch series
>> for this. But I can propose a separate patch series to implement the proposed
>> scheme.
>
> Right, IMHO, we can always add sysfs structure later, when its design is
> sane. What we should absolutely avoid is exposing something to userspace
> now and then try to hide it/replace it with something else and break
> userspace, which, as we all know, is a no-no, punishable by Linus coming
> to your house with a bat.
>
I am okay with this approach.

> :-) :-)
>
> --
> Regards/Gruss,
>     Boris.
>
> Sent from a fat crate under my desk. Formatting is fine.
> --
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