linux-kernel - Re: [PATCHv9 04/16] x86/cpu: Defer CR pinning setup until core initcall

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Message-ID: <c59aa7ac-62a6-45ec-b626-de518b25f7d9@intel.com>
Date: Fri, 1 Aug 2025 07:22:44 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Sohil Mehta <sohil.mehta@...el.com>, Thomas Gleixner
 <tglx@...utronix.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Kees Cook <kees@...nel.org>
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 X86-kernel <x86@...nel.org>
Subject: Re: [PATCHv9 04/16] x86/cpu: Defer CR pinning setup until core
 initcall

On 7/31/25 21:43, Sohil Mehta wrote:
...
> Could deferring enforcement lead to a scenario where we end up with
> different CR4 values on different CPUs? Maybe I am misinterpreting this
> and protecting against in-kernel errors is not a goal.

Sure, theoretically.

But if that's a concern, it can be checked at the time that enforcement
starts:

	for_each_online_cpu(cpu) {
		unsigned long cr4 = per_cpu(cpu_tlbstate.cr4, cpu);
		if ((cr4 & cr4_pinned_mask) == cr4_pinned_bits))
			continue;
		WARN("blah blah");
	}

Or use smp_call_function() to check each CPU's CR4 directly.

Or, the next time that CPU does a TLB flush that toggles X86_CR4_PGE,
it'll get the warning from the regular pinning path.

So, sure, this does widen the window during boot where a secondary CPU
might get a bad CR4 value, and it would make it harder to track down
where it happened. We _could_ print a pr_debug() message when the bit
gets cleared but not enforce things if anyone is super worried about this.

> In general, you want to delay the CR pinning enforcement until
> absolutely needed. I am curious about the motivation. I understand we
> should avoid doing it at arbitrary points in the boot. But,
> arch_cpu_finalize_init() and early_initcall() seem to be decent
> mileposts to me.
> 
> Are you anticipating that we would need to move setup_cr_pinning() again
> when another user similar to EFI shows up?
Yep.