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Subject: Re: [PATCH 1/4] X86: Update mmu_cr4_features during feature
 identification
To:     John Andersen <john.s.andersen@intel.com>, corbet@lwn.net,
        pbonzini@redhat.com, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, x86@kernel.org, hpa@zytor.com, shuah@kernel.org,
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        drjones@redhat.com, rick.p.edgecombe@intel.com,
        kristen@linux.intel.com
Cc:     vkuznets@redhat.com, wanpengli@tencent.com, jmattson@google.com,
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        gregkh@linuxfoundation.org, paulmck@kernel.org,
        pawan.kumar.gupta@linux.intel.com, jgross@suse.com,
        mike.kravetz@oracle.com, oneukum@suse.com, luto@kernel.org,
        peterz@infradead.org, fenghua.yu@intel.com,
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <539ad252-599f-1be5-3bde-196829929af2@intel.com>
Date:   Thu, 18 Jun 2020 07:09:15 -0700
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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.