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From: Toshi Kani <toshi.kani@hpe.com>
To: Borislav Petkov <bp@suse.de>
Cc: jgross@suse.com, mcgrof@suse.com, x86@kernel.org,
	linux-kernel@vger.kernel.org, paul.gortmaker@windriver.com,
	hpa@zytor.com, xen-devel@lists.xenproject.org,
	tglx@linutronix.de, mingo@kernel.org, elliott@hpe.com
Subject: Re: [PATCH v2 2/6] x86/mm/pat: Add pat_disable() interface
Date: Tue, 22 Mar 2016 15:40:45 -0600	[thread overview]
Message-ID: <1458682845.6393.614.camel__21180.3231308751$1458679776$gmane$org@hpe.com> (raw)
In-Reply-To: <20160322165944.GC5656@pd.tnic>

On Tue, 2016-03-22 at 17:59 +0100, Borislav Petkov wrote:
> On Wed, Mar 16, 2016 at 06:46:55PM -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
> > provide an interface that disables the OS to initialize PAT MSR.
> 
> 			prevents the OS from initializing the PAT MSR.

Right. Will do.

> > 
> > PAT MSR initialization must be done on all CPUs with the specific
> 
> s/with/using/

Ditto.

> > sequence of operations defined in Intel SDM.  This requires MTRR
> 				   ^
> 				  the
> 
> s/MTRR/MTRRs/

Ditto.

> > to be enabled since pat_init() is called as part of MTRR init
> > from mtrr_rendezvous_handler().
> > 
> > Change pat_disable() as the interface to disable the OS to initialize
> > PAT MSR, and set PAT table with pat_keep_handoff_state().  This
> > interface can be called when PAT initialization may not be performed.
> 
> This paragraph reads funky and I can't really parse what it is trying to
> say.

Sorry... Here is a retry:

Make pat_disable() as the interface that prevents the OS from initializing
the PAT MSR.  MTRR will call this interface when it cannot provide the SDM-
defined sequence to initialize PAT.

> > This also assures that pat_disable() called from pat_bsp_init()
> > to set PAT table properly when CPU does not support PAT.
> > 
 :
> >  
> > -static inline void pat_disable(const char *reason)
> > +/**
> > + * pat_disable() - Disable the OS to initialize PAT MSR
> 
> 			^^^^
> 
> Err, what? The function name can't be more clear.

Will change to "Prevent the OS from initializing the PAT MSR".

I wanted to clarify that "disable" does not mean to disable PAT MSR.

> > + *
> > + * This function disables the OS to initialize PAT MSR, and calls
> 
> 		    "prevents the OS from initializing the PAT MSR..."

Will do.

> > + * pat_keep_handoff_state() to set PAT table to the handoff state.
> 
> We can see what is calls. You're explaining *what* the code does instead
> of *why* again.

Right...

> > + */
> > +void pat_disable(const char *reason)
> >  {
> 
> Why aren't you checking __pat_enabled here?
> 
> 	if (!__pat_enabled)
> 		return;

pat_keep_handoff_state() is a no-op after the initial call, but I agree
that having this check is better.  Will do.

> You can save yourself the other guards in that function, especially that
> pr_err() below.

The pr_err() below is for a difference case -- PAT is enabled, but a call
is made to disable it after pat_init() is called.  We cannot allow this
case.

> > +	if (boot_cpu_done) {
> > +		pr_err("x86/PAT: PAT cannot be disabled after
> > initialization "
> > +		       "(attempting: %s)\n", reason);
> 
> Please integrate checkpatch.pl into your patch creation workflow as it
> sometimes has valid complaints:
> 
> WARNING: quoted string split across lines
> #79: FILE: arch/x86/mm/pat.c:55:
> +               pr_err("x86/PAT: PAT cannot be disabled after
> initialization "
> +                      "(attempting: %s)\n", reason);

I've run checkpatch.pl and thought it was OK to have this warning (instead
of a >80 warning) since the error message part was not split.  The
"attempting" part is for debugging and its string is passed from the
caller. 

> More to the point: why do we need that pr_err() call? What is that
> supposed to tell the user?
> 
> I think it is more for the programmer to catch wrong use of
> pat_disable() and then it should be WARN_ONCE() or so...

Yes, this case is for the programmer to catch wrong use.  I will change it
to use WARN_ONCE() and remove the "(attempting: %s)\n" part of the message.

> > +		return;
> > +	}
> > +
> >  	__pat_enabled = 0;
> >  	pr_info("x86/PAT: %s\n", reason);
> > +
> > +	pat_keep_handoff_state();
> >  }
> >  
> >  static int __init nopat(char *str)
> > @@ -202,7 +217,7 @@ static void pat_bsp_init(u64 pat)
> >  {
> >  	u64 tmp_pat;
> >  
> > -	if (!cpu_has_pat) {
> > +	if (!boot_cpu_has(X86_FEATURE_PAT)) {
> >  		pat_disable("PAT not supported by CPU.");
> >  		return;
> >  	}
> > @@ -220,7 +235,7 @@ static void pat_bsp_init(u64 pat)
> >  
> >  static void pat_ap_init(u64 pat)
> >  {
> > -	if (!cpu_has_pat) {
> > +	if (!boot_cpu_has(X86_FEATURE_PAT)) {
> >  		/*
> >  		 * If this happens we are on a secondary CPU, but
> > switched to
> >  		 * PAT on the boot CPU. We have no way to undo PAT.
> 
> Those last two hunks are unrelated changes and should be a separate
> patch.

Will do.

Thanks,
-Toshi

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  parent reply	other threads:[~2016-03-22 20:48 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-17  0:46 Toshi Kani
2016-03-17  0:46 ` [PATCH v2 3/6] x86/mtrr: Fix Xorg crashes in Qemu sessions Toshi Kani
2016-03-17  0:46 ` [PATCH v2 4/6] x86/mtrr: Fix PAT init handling when MTRR MSR is disabled Toshi Kani
2016-03-17  0:46 ` [PATCH v2 5/6] x86/xen, pat: Remove PAT table init code from Xen Toshi Kani
2016-03-17  0:46 ` [PATCH v2 6/6] x86/pat: Document PAT initializations Toshi Kani
2016-03-22 16:59 ` [PATCH v2 2/6] x86/mm/pat: Add pat_disable() interface Borislav Petkov
     [not found] ` <1458175619-32206-2-git-send-email-toshi.kani@hpe.com>
2016-03-22 17:00   ` [PATCH v2 3/6] x86/mtrr: Fix Xorg crashes in Qemu sessions Borislav Petkov
     [not found]   ` <20160322170047.GD5656@pd.tnic>
2016-03-22 21:53     ` Toshi Kani
     [not found]     ` <1458683610.6393.624.camel@hpe.com>
2016-03-23  8:44       ` Borislav Petkov
     [not found]       ` <20160323084437.GB8031@pd.tnic>
2016-03-23 15:53         ` Toshi Kani
     [not found]         ` <1458748393.6393.653.camel@hpe.com>
2016-03-23 21:47           ` Toshi Kani
     [not found] ` <1458175619-32206-3-git-send-email-toshi.kani@hpe.com>
2016-03-22 17:01   ` [PATCH v2 4/6] x86/mtrr: Fix PAT init handling when MTRR MSR is disabled Borislav Petkov
     [not found]   ` <20160322170135.GE5656@pd.tnic>
2016-03-22 22:02     ` Toshi Kani
     [not found] ` <1458175619-32206-4-git-send-email-toshi.kani@hpe.com>
2016-03-22 17:02   ` [PATCH v2 5/6] x86/xen, pat: Remove PAT table init code from Xen Borislav Petkov
     [not found]   ` <20160322170206.GF5656@pd.tnic>
2016-03-23  6:08     ` Juergen Gross
     [not found] ` <1458175619-32206-5-git-send-email-toshi.kani@hpe.com>
2016-03-22 17:02   ` [PATCH v2 6/6] x86/pat: Document PAT initializations Borislav Petkov
     [not found]   ` <20160322170222.GG5656@pd.tnic>
2016-03-22 22:15     ` Toshi Kani
     [not found] ` <20160322165944.GC5656@pd.tnic>
2016-03-22 21:40   ` Toshi Kani [this message]
     [not found]   ` <1458682845.6393.614.camel@hpe.com>
2016-03-23  8:51     ` [PATCH v2 2/6] x86/mm/pat: Add pat_disable() interface Borislav Petkov
     [not found]     ` <20160323085141.GC8031@pd.tnic>
2016-03-23 15:49       ` Toshi Kani

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