linux-kernel - Re: [PATCH v1] x86/smp: Set up exception handling before cr4

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Message-ID: <37de06e1-aae4-4ebd-ac93-1846ee4cd91e@intel.com>
Date: Sun, 8 Feb 2026 23:28:41 -0800
From: Sohil Mehta <sohil.mehta@...el.com>
To: Xin Li <xin@...or.com>, Dave Hansen <dave.hansen@...el.com>
CC: <linux-kernel@...r.kernel.org>, <tglx@...utronix.de>, <mingo@...hat.com>,
	<bp@...en8.de>, <dave.hansen@...ux.intel.com>, <x86@...nel.org>,
	<hpa@...or.com>, <peterz@...radead.org>, <andrew.cooper3@...rix.com>,
	<nikunj@....com>, <thomas.lendacky@....com>, <seanjc@...gle.com>,
	<stable@...r.kernel.org>
Subject: Re: [PATCH v1] x86/smp: Set up exception handling before cr4_init()

On 2/8/2026 11:02 AM, Xin Li wrote:

> 
> I’m curious why cr4_init() is not part of the following cpu_init()? IOW,
> why does it need to be called so early in the existing code?
> 

The name cpu_init() is misleading. Most of the pinned features don't get
initialized in cpu_init(). They are set up slightly later:

start_secondary()
  ap_starting()
    identify_secondary_cpu()
      identify_cpu()

The original reason for writing CR4 early on APs probably originates in
commit c7ad5ad297e6 ("x86/mm/64: Initialize CR4.PCIDE early"). Then,
when CR pinning was introduced, it was a global system-wide concept. So,
the pinned bits had to be programmed when the first write to CR4 happened.

> 
>>
>> I _really_ think we need a defined per-cpu point where pinning comes
>> into effect. Marking the CPU online is one idea.
>>
>> Thoughts?
> 

I think this approach could work. It should cover APs as well as hotplug
CPUs that come online later.

> It seems a good fit.  Just that {on,off}line() are not called on BSP (not
> a real problem).
> 

The BSP is marked online in boot_cpu_init()->set_cpu_online(). So, it
should be covered as well.

> Question is that who would work on it ;) ?

I think Dave already posted the patch for it here.
https://lore.kernel.org/lkml/02df7890-83c2-4047-8c88-46fbc6e0a892@intel.com/

I will test that out to confirm that it doesn't mess up some implicit
behavior.