Re: [PATCH 1/4] X86: Update mmu_cr4_features during feature identification

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Overall this looks pretty good.  For all the maintainers on cc, we try
to do internal reviews of these before they're submitted.  This one got
missed, sorry about that.

On 6/17/20 12:07 PM, John Andersen wrote:
> In identify_cpu when setting up SMEP/SMAP/UMIP call
> cr4_set_bits_and_update_boot instead of cr4_set_bits. This ensures that
> mmu_cr4_features contains those bits, and does not disable those
> protections when in hibernation asm.

When I'm writing comments, I try to use parenthesis for functions(),
which leaves variable_names plain.

I also try not to dive directly into the function names.  This
description assumes that the reader knows the subtle difference between
cr4_set_bits_and_update_boot() and of cr4_set_bits().  A sentence or two
of background here can save a reviewer a dive into the source code.

> setup_arch updates mmu_cr4_features to save what identified features are
> supported for later use in hibernation asm when cr4 needs to be modified
> to toggle PGE. cr4 writes happen in restore_image and restore_registers.
> setup_arch occurs before identify_cpu, this leads to mmu_cr4_features
> not containing some of the cr4 features which were enabled via
> identify_cpu when hibernation asm is executed.

This fails to address the bigger picture.  I assume you end up wanting
this because without it hibernation is not compatible with CR pinning.
Shouldn't that be mentioned?

I also wonder why we even need two classes of cr4_set_bits().  Are there
features we *want* to disable before entering the hibernation assembly?

For instance, why not leave MCE enabled in there?  What about PCIDs or
OSPKE?  Does it hurt?

> On CPU bringup when cr4_set_bits_and_update_boot is called 
> mmu_cr4_features will now be written to. For the boot CPU, the
> __ro_after_init on mmu_cr4_features does not cause a fault. However, 
> __ro_after_init was removed due to it triggering faults on non-boot 
> CPUs
Before this patch, cr4_set_bits_and_update_boot() was only ever called
during init.  But, after this patch, it gets called later in boot and
causes problems.  We're surely not making _real_ updates to it, right?
In that case the writes are superfluous and we would be better off just
not writing to it (and retaining __ro_after_init) rather than allowing
superfluous writes.



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