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

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Message-ID: <6a71fb30-32f2-4847-b4da-e52b67433ce0@intel.com>
Date: Tue, 7 Oct 2025 16:05:30 -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 10/7/2025 10:23 AM, Edgecombe, Rick P wrote:

> 
> Why is only set_virtual_address_map problematic? Some of the other ones get
> called after boot by a bunch of modules by the looks of it.
> 

AFAIU, efi_enter_virtual_mode()->set_virtual_address_map maps the
runtime services from physical mode into virtual mode.

After that, all the other runtime services can get called using virtual
addressing. I can find out more if you still have concerns.

>> @@ -476,8 +476,8 @@ void cr4_init(void)
>>  
>>  	if (boot_cpu_has(X86_FEATURE_PCID))
>>  		cr4 |= X86_CR4_PCIDE;
>> -	if (static_branch_likely(&cr_pinning))
>> -		cr4 = (cr4 & ~cr4_pinned_mask) | cr4_pinned_bits;
>> +
>> +	cr4 = (cr4 & ~cr4_pinned_mask) | cr4_pinned_bits;
> 
> 
> Can you explain why this change is needed? It relies on cr4_pinned_bits to be
> already set, and kind of is "enforcement", but no longer depends on
> enable_cr_pinning() being called.
> 

cr4_init() is only called from APs during bring up. The pinned bits are
saved on the BSP and then used to program the CR4 on the APs. It is
independent of pinning *enforcement* which warns when these bits get
modified.

> 
>>  
>>  	__write_cr4(cr4);
>>