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Re: [PATCH v2 2/6] x86/mm/pat: Add pat_disable() interface

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  Re: [PATCH v2 2/6] x86/mm/pat: Add pat_disable() interface Borislav Petkov <bp@suse.de> - 2016-03-22 18:10 +0100
    Re: [PATCH v2 2/6] x86/mm/pat: Add pat_disable() interface Toshi Kani <toshi.kani@hpe.com> - 2016-03-22 21:50 +0100
      Re: [PATCH v2 2/6] x86/mm/pat: Add pat_disable() interface Borislav Petkov <bp@suse.de> - 2016-03-23 10:00 +0100
        Re: [PATCH v2 2/6] x86/mm/pat: Add pat_disable() interface Toshi Kani <toshi.kani@hpe.com> - 2016-03-23 16:00 +0100

#1362860 — Re: [PATCH v2 2/6] x86/mm/pat: Add pat_disable() interface

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.

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

s/with/using/

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

s/MTRR/MTRRs/

> 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.

> This also assures that pat_disable() called from pat_bsp_init()
> to set PAT table properly when CPU does not support PAT.
> 
> Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Luis R. Rodriguez <mcgrof@suse.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: Robert Elliott <elliott@hpe.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> ---
>  arch/x86/include/asm/pat.h |    1 +
>  arch/x86/mm/pat.c          |   21 ++++++++++++++++++---
>  2 files changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pat.h b/arch/x86/include/asm/pat.h
> index ca6c228..016142b 100644
> --- a/arch/x86/include/asm/pat.h
> +++ b/arch/x86/include/asm/pat.h
> @@ -5,6 +5,7 @@
>  #include <asm/pgtable_types.h>
>  
>  bool pat_enabled(void);
> +void pat_disable(const char *reason);
>  extern void pat_init(void);
>  void pat_init_cache_modes(u64);
>  
> diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
> index e0a34b0..48d1619 100644
> --- a/arch/x86/mm/pat.c
> +++ b/arch/x86/mm/pat.c
> @@ -40,11 +40,26 @@
>  static bool boot_cpu_done;
>  
>  static int __read_mostly __pat_enabled = IS_ENABLED(CONFIG_X86_PAT);
> +static void pat_keep_handoff_state(void);
>  
> -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.

> + *
> + * This function disables the OS to initialize PAT MSR, and calls

		    "prevents the OS from initializing the PAT MSR..."

> + * 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.

> + */
> +void pat_disable(const char *reason)
>  {

Why aren't you checking __pat_enabled here?

	if (!__pat_enabled)
		return;

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

> +	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);

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...

> +		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.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
-- 

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#1362968

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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#1363263

On Tue, Mar 22, 2016 at 03:40:45PM -0600, Toshi Kani wrote:
> Will change to "Prevent the OS from initializing the PAT MSR".
> 
> I wanted to clarify that "disable" does not mean to disable PAT MSR.

How do you "disable PAT MSR" ?

I think you're overdocumenting this. pat_disable() is as clear as day
what it does. It doesn't need any commenting...

> 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. 

We always put the quoted strings on a single line for easier grepping.
Forget the 80-cols rule.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
-- 

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#1363476

On Wed, 2016-03-23 at 09:51 +0100, Borislav Petkov wrote:
> On Tue, Mar 22, 2016 at 03:40:45PM -0600, Toshi Kani wrote:
> > Will change to "Prevent the OS from initializing the PAT MSR".
> > 
> > I wanted to clarify that "disable" does not mean to disable PAT MSR.
> 
> How do you "disable PAT MSR" ?

We can't, but I thought not everyone knows how it works...

> I think you're overdocumenting this. pat_disable() is as clear as day
> what it does. It doesn't need any commenting...

Right, maybe I am just paranoid.  I will remove the comment as you
suggested.

> > 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. 
> 
> We always put the quoted strings on a single line for easier grepping.
> Forget the 80-cols rule.

OK.

Thanks,
-Toshi

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