Re: [PATCH v1] x86/smp: Set up exception handling before cr4_init()

[Sohil Mehta <sohil.mehta@intel.com>]

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.