Re: [PATCH v10 05/15] x86/cpu: Defer CR pinning enforcement until late_initcall()

[Sohil Mehta]

On 10/6/2025 11:51 PM, Sohil Mehta wrote:
> Save the pinned bits while initializing the boot CPU because they are
> needed later to program the value on APs when they come up.
> 

Because we are deferring CR pinning, there is no need to program the APs
with the pinned bits. The pinned bits would get enabled during AP bring
up like the rest of CR4 features that are not pinned. This patch can be
simplified to:

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 61ab332eaf73..d041f04c1969 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -476,8 +476,6 @@ 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;

        __write_cr4(cr4);

@@ -486,15 +484,17 @@ void cr4_init(void)
 }

 /*
- * Once CPU feature detection is finished (and boot params have been
- * parsed), record any of the sensitive CR bits that are set, and
- * enable CR pinning.
+ * Before userspace starts, record any of the sensitive CR bits that
+ * are set, and enable CR pinning.
  */
-static void __init setup_cr_pinning(void)
+static int __init setup_cr_pinning(void)
 {
        cr4_pinned_bits = this_cpu_read(cpu_tlbstate.cr4) & cr4_pinned_mask;
        static_key_enable(&cr_pinning.key);
+
+       return 0;
 }
+late_initcall(setup_cr_pinning);

 static __init int x86_nofsgsbase_setup(char *arg)
 {
@@ -2119,7 +2119,6 @@ static __init void identify_boot_cpu(void)
        enable_sep_cpu();
 #endif
        cpu_detect_tlb(&boot_cpu_data);
-       setup_cr_pinning();

        tsx_init();
        tdx_init();