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

[Toshi Kani <toshi.kani@xxxxxxx>]

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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