linux-kernel - Re: [PATCH v3] x86/mce: Honour bios-set CMCI threshold

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Message-ID: <507ED31B.7010402@linux.vnet.ibm.com>
Date:	Wed, 17 Oct 2012 21:17:39 +0530
From:	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:	Borislav Petkov <bp@...en8.de>, tony.luck@...el.com,
	gong.chen@...ux.intel.com, ananth@...ibm.com,
	masbock@...ux.vnet.ibm.com, x86@...nel.org,
	linux-kernel@...r.kernel.org, bp@...64.org, lcm@...ibm.com,
	andi@...stfloor.org, hpa@...or.com, tglx@...utronix.de,
	mingo@...hat.com, gregkh@...e.de, linux-edac@...r.kernel.org
Subject: Re: [PATCH v3] x86/mce: Honour bios-set CMCI threshold

On 10/17/2012 06:39 PM, Borislav Petkov wrote:
> On Wed, Oct 17, 2012 at 04:57:30PM +0530, Naveen N. Rao wrote:
>> On 10/17/2012 04:29 PM, Borislav Petkov wrote:
>>>>>
>>>>> +static struct dev_ext_attribute dev_attr_bios_cmci_threshold = {
>>>>> +	__ATTR(bios_cmci_threshold, 0444, device_show_int, NULL),
>>>>> +	&mce_bios_cmci_threshold
>>>
>>> Ok, I just noticed this (we must've missed it during review) but why is
>>> this read-only? If it has to be read-only, why do we have a node for
>>> this in sysfs instead of simply issuing the printk statements below and
>>> people who are interested in this, can grep dmesg?
>>
>> This was added so that user-space tools could find out if we're
>> using thresholds for CMCI.
>
> That I figured out.
>
> What I can't figure out is why userspace tools need to know that - the
> fact that some MCI_CTL2 has a 0 CMCI threshold because BIOS forgot to
> set it correctly? IOW, this is a rather evolved workaround for b0rked
> BIOS (the gazillionth BIOS f*ckup, btw if someone is counting :)) and,
> on top of that, we have a read-only, special sysfs node which is pretty
> useless to me.
>
> Why?
>

This is not about the CMCI threshold being 0, but rather about the 
threshold not being 1. Linux used to program a threshold of 1 always but 
with this boot option, a firmware-set non-zero threshold is honoured.

Userspace tools need this sysfs attribute so they know how to react on 
receipt of a corrected error event: whether this is the first event or 
if such events have already been threshold-ed.


Regards,
Naveen

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