linux-kernel - Re: [PATCH] x86: Quark: Flush TLB via CR3 not CR4.PGE in setup

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Date:	Thu, 25 Sep 2014 13:49:11 -0300
From:	Henrique de Moraes Holschuh <hmh@....eng.br>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Bryan O'Donoghue <pure.logic@...us-software.ie>, hpa@...or.com,
	mingo@...hat.com, tglx@...utronix.de, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Quark: Flush TLB via CR3 not CR4.PGE in setup_arch()

On Thu, 25 Sep 2014, Ingo Molnar wrote:
> > If we're adding a comment though the first thing the comment ought to say is
> > what the code does for everybody else - stuff CR3 and flush the TLB, then it
> > should comment on the exception for Quark.
> > 
> > /*
> >  * Locate the page directory and flush the TLB.
> >  *
> >  * On Quark CPUs we still have the PGE bit set so
> >  * __flush_tlb_all() is not yet doing what it says - but
> >  * accidentally we have a cr3 flush here which is what is
> >  * needed - so there's no need to add a Quark quirk here.
> >  */
> > 
> > ?
> 
> Yeah, fair enough. You can even put the latter in parentheses, to 
> signal that it's all a rare case.

I'd have mentioned "erratum" there, otherwise people won't understand why
the PGE bit being set would be a problem in the first place.

Something like:

/*
 * Locate the page directory and flush the TLB.
 *
 * On Quark X1000 CPUs we still have the PGE bit incorrectly set
 * due to a processor erratum, so __flush_tlb_all() is not yet
 * doing what it says.  Fortunately we have a cr3 flush here,
 * which is what is needed in this processor to flush TLBs, so
 * there's no need to add a Quark X1000 quirk here.
 */

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh
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