Re: [PATCH 1/2] x86/mm/pat: Change pat_disable() to emulate PAT table

From: Toshi Kani
Date: Tue Mar 15 2016 - 17:09:38 EST

On Tue, 2016-03-15 at 12:00 +0100, Borislav Petkov wrote:
> On Mon, Mar 14, 2016 at 03:37:23PM -0600, Toshi Kani wrote:
> > Your patch is a simplified version of mine. ÂSo, yes, it fixes the
> > Paul's issue, but it does not address other issues that my patchset
> > also addressed. ÂIn specific, I think your patch has the following
> > issues.
> You couldnt've structured your reply better: remember how I split a
> convoluted patch of yours already? A patch which was trying to do a
> bunch of things in one go.
> The situation here is the same. You need to do *one* *logical*
> *non-trivial* thing in a patch. If there's something else that needs to
> be done, add it in a *separate* patch which explains why that new change
> is needed.

Got it!

> > - pat_disable() is now callable from other modules. So, it needs to
> > check withÂboot_cpu_done. We cannot disable PAT once it is initialized.
> That should be a separate patch which explains *why* the change is being
> done.
> > - mtrr_bp_init() needs to check with mtrr_enabled() when it
> > callsÂmtrr_pat_setup_bp(). Otherwise, PAT is left initialized on BSP
> > only when MTRR is disabled by its MSR. In your patch, mtrr_bp_init()
> > calls pat_setup() again, but it does not help since boot_cpu_done is
> > set.
> The code which you carved out from get_mtrr_state() didn't check
> mtrr_enabled() before. That needs to be another patch *again* with
> explanations.
> > - When PAT is disabled in CPU feature, pat_bsp_init() calls
> > pat_disable() and returns. However, it does not initialize a PAT table
> > by calling pat_init_cache_modes().
> Yet another patch.
> > - When CONFIG_MTRR is unset, it does not call pat_setup().
> Aaaand... can you guess what I'm going to say here?
> I hope it is coming across as I intend it: please use my hunk to do a
> single fix and then prepare all those changes above in separate patches
> with explanations:

Unfortunately, this single fix will break Xen. ÂSo, I think we will need to
make a few enhancements first before making the fix.

> "Problem is A. We need to do B. I'm doing it/I'm doing C because."
> Ok?

Yes, I will try to separate the patches to change one logical thing at a