linux-kernel - Re: [PATCH v10 05/15] x86/cpu: Defer CR pinning enforcement until late

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Message-ID: <5e259c5d-3367-4efb-8291-69fbbcb0003c@intel.com>
Date: Wed, 15 Oct 2025 14:17:19 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "tglx@...utronix.de"
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Subject: Re: [PATCH v10 05/15] x86/cpu: Defer CR pinning enforcement until
 late_initcall()

On 10/10/2025 1:45 PM, Sohil Mehta wrote:
> 
> As we discussed in another thread, CR pinning has expanded beyond the
> original security related bits. They have become bits that are never
> expected to be modified once initialized. I wonder whether we could run
> into issues if the initial CR4 value on the APs doesn't have one of the
> pinned bits set. From a cursory look, everything should be fine (except
> maybe FRED). I could give it a try.
> 

I tried this. Getting rid of the cr4_pinned_bits setting during
cr_init() seems to be working fine.

Xin says that there may be an existing issue with FRED, as CR4.FRED is
set before programming the FRED config MSRs in
cpu_init_fred_exceptions(). Any exceptions during that brief window,
though unlikely, would cause a triple fault. I think not setting
CR4.FRED might help the issue, but, I am not sure. I'll let Xin or Peter
evaluate this.


> But, is there a preference here? There is no additional cost of setting
> the pinned bits because we definitely need to program X86_CR4_PCIDE. Do
> we set the pinned bits along with that, or wait for the AP to go through
> the init flow and set them one by one?
> 

As we are planning to defer CR pinning enforcement, I am inclining
towards getting rid of the following check in cr4_init().

	if (static_branch_likely(&cr_pinning))
		cr4 = (cr4 & ~cr4_pinned_mask) | cr4_pinned_bits;

AFAIU, this change shouldn't harm FRED. Resolving the existing FRED
issue can be done in a separate patch.