linux-kernel - Re: [PATCH v2 1/6] x86/mm/pat: Change PAT to support non-default PAT MSR

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Date:	Tue, 22 Mar 2016 12:35:19 -0600
From:	Toshi Kani <toshi.kani@....com>
To:	Borislav Petkov <bp@...e.de>
Cc:	mingo@...nel.org, hpa@...or.com, tglx@...utronix.de,
	mcgrof@...e.com, jgross@...e.com, paul.gortmaker@...driver.com,
	konrad.wilk@...cle.com, elliott@....com, x86@...nel.org,
	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/6] x86/mm/pat: Change PAT to support non-default
 PAT MSR

On Tue, 2016-03-22 at 17:57 +0100, Borislav Petkov wrote:
> $Subject is misleading - there's no non-default PAT MSR - the setting is
> non-default.

Right.  Will change to "Add support of non-default PAT MSR setting at
handoff".

> On Wed, Mar 16, 2016 at 06:44:57PM -0600, Toshi Kani wrote:
> > In preparation to fix a regression caused by 'commit 9cd25aac1f44
> > ("x86/mm/pat: Emulate PAT when it is disabled")', PAT needs to
> > support a case that PAT MSR is initialized with a non-default
> > value.
> > 
> > When pat_init() is called in PAT disable state, it initializes
> 
> 		  is called and PAT is disabled

Will do.

> > PAT table with the BIOS default value. Xen, however, sets PAT MSR
> > with a non-default value to enable WC. This causes inconsistency
> > between PAT table and PAT MSR when PAT is set to disable on Xen.
> > 
> > Change pat_init() to handle the PAT disable cases properly.  Add
> > pat_keep_handoff_state() to handle two cases when PAT is set to
> > disable.
> >  1. CPU supports PAT: Set PAT table to be consistent with PAT MSR.
> >  2. CPU does not support PAT: Set PAT table to be consistent with
> >     PWT and PCD bits in a PTE.
> > 
 :
> > +/**
> > + * pat_keep_handoff_state - Set PAT table to the handoff state
> > + *
> > + * This function keeps PAT in the BIOS handoff state. When CPU
> > supports
> > + * PAT, it sets PAT table to be consistent with PAT MSR. When CPU does
> > not
> > + * support PAT, it emulates PAT by setting PAT table consistent with
> > PWT
> > + * and PCD bits in a PTE.
> > + *
> > + * The PAT table is global to all CPUs, which is initialized once at
> > + * boot-time. Any subsequent calls to this function have no effect.
> > + */
> > +static void pat_keep_handoff_state(void)
> 
> Static function, no need for "pat_" prefix. Also, no need for the
> kernel-doc comment.
> 
> Also, no need for all that handoff nomenclature etc, just call it
> setup_pat(). Because it does exactly that - it sets up the PAT bits
> unconditionally, regardless of enabled or not.

I'd like to make it clear that this function does not set PAT MSR, unlike
what pat_init() does.  When CPU supports PAT, it keeps PAT MSR in whatever
the setting at handoff, and initializes PAT table to match with this
setting.

I am open to a better name, but I am afraid that setup_pat() can be
confusing as if it sets PAT MSR.

> >  {
> > -	u64 pat;
> > -	struct cpuinfo_x86 *c = &boot_cpu_data;
> > +	u64 pat = 0;
> > +	static int set_handoff_done;
> 
> s/set_handoff_done/pat_setup_done/

I will match it with a func name once we decided.

Thanks,
-Toshi