From: Dave Hansen <dave.hansen@intel.com>
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,
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Subject: Re: [PATCH 1/4] X86: Update mmu_cr4_features during feature identification
Date: Thu, 18 Jun 2020 07:09:15 -0700 [thread overview]
Message-ID: <539ad252-599f-1be5-3bde-196829929af2@intel.com> (raw)
In-Reply-To: <20200617190757.27081-2-john.s.andersen@intel.com>
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.
next prev parent reply other threads:[~2020-06-18 14:09 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-17 19:07 [PATCH 0/4] Paravirtualized Control Register pinning John Andersen
2020-06-17 19:07 ` [PATCH 1/4] X86: Update mmu_cr4_features during feature identification John Andersen
2020-06-18 14:09 ` Dave Hansen [this message]
2020-06-17 19:07 ` [PATCH 2/4] KVM: x86: Introduce paravirt feature CR0/CR4 pinning John Andersen
2020-06-18 14:18 ` Dave Hansen
2020-06-18 14:43 ` Andersen, John
2020-06-18 14:51 ` Dave Hansen
2020-07-07 21:12 ` Sean Christopherson
2020-07-07 21:48 ` Dave Hansen
2020-07-07 21:51 ` Paolo Bonzini
2020-07-09 15:44 ` Andersen, John
2020-07-09 15:56 ` Dave Hansen
2020-07-09 16:07 ` Andy Lutomirski
2020-07-09 16:22 ` Dave Hansen
2020-07-09 16:27 ` Andy Lutomirski
2020-07-14 5:36 ` Andersen, John, Arvind Sankar
2020-07-14 5:39 ` Andersen, John
2020-07-15 4:41 ` Sean Christopherson
2020-07-15 19:58 ` Andersen, John
2020-07-09 23:37 ` Kees Cook
2020-06-17 19:07 ` [PATCH 3/4] selftests: kvm: add test for CR pinning with SMM John Andersen
2020-06-17 19:07 ` [PATCH 4/4] X86: Use KVM CR pin MSRs John Andersen
2020-06-18 14:41 ` Dave Hansen
2020-06-18 15:26 ` Andersen, John
2020-06-18 15:38 ` Dave Hansen
2020-06-18 15:49 ` Andersen, John
2020-06-20 5:13 ` Andy Lutomirski
2020-06-23 20:03 ` Andersen, John
2020-07-03 21:48 ` Andersen, John
2020-07-04 15:11 ` Arvind Sankar
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