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

[Dave Hansen]

On 7/31/25 16:45, Sohil Mehta wrote:
> On 7/9/2025 10:00 AM, Dave Hansen wrote:
>> On 7/7/25 01:03, Kirill A. Shutemov wrote:
>>> Instead of moving setup_cr_pinning() below efi_enter_virtual_mode() in
>>> arch_cpu_finalize_init(), defer it until core initcall.
>> What are the side effects of this move? Are there other benefits? What
>> are the risks?
>>
> Picking this up from Kirill.. Reevaluating this, core_initcall() seems
> too late for setup_cr_pinning().
> 
> We need to have CR pinning completed, and the associated static key
> enabled before AP bring up. start_secondary()->cr4_init() depends on the
> cr_pinning static key to initialize CR4 for APs.

Sure, if you leave cr4_init() completely as-is.

'cr4_pinned_bits' should be set by the boot CPU. Secondary CPUs should
also read 'cr4_pinned_bits' when setting up their own cr4's,
unconditionally, independent of 'cr_pinning'.

The thing I think we should change is the pinning _enforcement_. The
easiest way to do that is to remove the static_branch_likely() in
cr4_init() and then delay flipping the static branch until just before
userspace starts.

Basically, split up the:

static void __init setup_cr_pinning(void)
{
    cr4_pinned_bits = this_cpu_read(cpu_tlbstate.cr4) & cr4_pinned_mask;
    static_key_enable(&cr_pinning.key);
}

code into its two logical pieces:

 1. Populate 'cr4_pinned_bits' from the boot CPU so the secondaries can
    use it
 2. Enable the static key so pinning enforcement is enabled