Subject: Re: [PATCH V4] powerpc/85xx: Add machine check handler to fix PCIe erratum on mpc85xx

From: Scott Wood <[email-masked]>
Date: Tue, 5 Mar 2013 12:47:42 -0600

On 03/05/2013 04:12:30 AM, Jia Hongtao-B38951 wrote:
>=20
>=20
> > -----Original Message-----
> > From: Wood Scott-B07421
> > Sent: Tuesday, March 05, 2013 7:46 AM
> > To: Stuart Yoder
> > Cc: Jia Hongtao-B38951; [email-masked]; Kumar Gala
> > Subject: Re: [PATCH V4] powerpc/85xx: Add machine check handler to =20
> fix
> > PCIe erratum on mpc85xx
> >
> > On 03/04/2013 10:16:10 AM, Stuart Yoder wrote:
> > > On Mon, Mar 4, 2013 at 2:40 AM, Jia Hongtao <[email-masked]>
> > > wrote:
> > > > A PCIe erratum of mpc85xx may causes a core hang when a link of =20
> PCIe
> > > > goes down. when the link goes down, Non-posted transactions =20
> issued
> > > > via the ATMU requiring completion result in an instruction =20
> stall.
> > > > At the same time a machine-check exception is generated to the =20
> core
> > > > to allow further processing by the handler. We implements the
> > > handler
> > > > which skips the instruction caused the stall.
> > >
> > > Can you explain at a high level how just skipping an instruction
> > > solves
> > > anything?   If you just skip a load/store and continue like =20
> nothing is
> > > wrong, isn't your system possibly in a really bad state.
> >
> > If the instruction was a load, we probably at least want to fill the
> > destination register with 0xffffffff or similar.
>=20
> You discuss this with Liu Shuo about a year ago.
> here is the log:
>=20
> "
> On 02/01/2012 02:18 AM, [email-masked] wrote:
> > v3 : Skip the instruction only. Don't access the user space memory =20
> in
> >      mechine check.
>=20
> It may be the least bad option for now, but be aware that there's a
> small chance that this will cause a leak of sensitive information =20
> (such
> as a piece of a crypto key that happened to be sitting in the register
> to be loaded into).

Yes, that's (one reason) why you'd want to fill in a known value.  Note =20
the "for now". :-)

-Scott=