Subject: Re: [PATCH] [3/4] x86: MCE: Improve mce_get_rip
From: Huang Ying <ying.huang@intel.com>
To: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Cc: Andi Kleen <andi@firstfloor.org>, "hpa@zytor.com" <hpa@zytor.com>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       "mingo@elte.hu" <mingo@elte.hu>,
       "tglx@linutronix.de" <tglx@linutronix.de>
In-Reply-To: <49F15922.5090704@jp.fujitsu.com>
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	 <20090407150656.43E161D046D@basil.firstfloor.org>
	 <49DC5D11.4060505@jp.fujitsu.com>
	 <1240479833.6842.554.camel@yhuang-dev.sh.intel.com>
	 <49F15922.5090704@jp.fujitsu.com>
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Date: Fri, 24 Apr 2009 14:35:11 +0800
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On Fri, 2009-04-24 at 14:16 +0800, Hidetoshi Seto wrote:
> Huang Ying wrote:
> > Add some description for the patch, hope that to be more clear.
> >=20
> > Best Regards,
> > Huang Ying
> > --------------------------------------------->
> > mce_get_rip() is used to get IP when MCE is generated, usually from
> > the stack. But the IP on the stack is not always valid.
> > MCG_STATUS_RIPV indicates program can restart from the IP on the stack,
> > so if it is set, the IP is valid. MCG_STATUS_EIPV indicate IP on the
> > stack is directly associated with the error, so if it is set, the IP
> > is valid too.
> >=20
> > In current implementation, no IP will be returned (and then reported)
> > if MCG_STATUS_RIPV is not set and MCG_STATUS_EIPV is set. This patch
> > fixes this issue by returning IP on the stack when MCG_STATUS_EIPV is
> > set.
> >=20
> > In some CPU, a MSR (rip_msr) provides another way to get IP when MCE
> > is generated. This is used by mce_get_rip() too.
> >=20
> > There is no MSR for CS, in current implementation, if rip_msr is used
> > to get IP, reported CS is set to 0. But in fact, the CS on the stack
> > can be trusted if MCG_STATUS_RIPV or MCG_STATUS_EIPV is set. This
> > patch fixes this issue by keeping reported CS when rip_msr is used.
>=20
> So the bug is in short:
>  In some cases no IP/CS reported even there were valid records.
> Right?
>=20
> Then in other words it will mean lost of error information, that is not
> good for error investigation.
>=20
> One question is: if (RIPV,EIPV) =3D (0,0), then is the IP on the stack
> really invalid value, or is it still point IP when MCE is generated?
> I suppose it is not invalid.  If a processor encounters MCE and if it
> is not sure what happened, then it will store the IP on the stack,
> indicating neither of flags.
>=20
> If this supposition is correct, the best way is pick the value on
> the stack unconditionally, and record valid flags together.

According to spec, the IP on stack can be not related to MCE if
(RIPV,EIPV) =3D (0,0). So it is meaningless to report them. If you report
them unconditionally, you just push the logic to user space or
administrator.

Best Regards,
Huang Ying


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