[PATCH v1 5/5] x86/cpu: Don't reset boot CPU cpuinfo in identify_cpu()

[Ihor Solodrai <ihor.solodrai@linux.dev>]

identify_cpu() used to rebuild c->x86_capability from scratch:
init_cpu_info() memsets the capability array to zero and
identify_cpu() then re-reads CPUID. On the boot CPU identify_cpu()
runs after local_irq_enable() and before alternatives are patched. An
interrupt delivered in the window between the memset() and
get_cpu_cap() therefore will observe cleared x86_capability [1].

However the boot CPU's capabilities have already been scanned, with
interrupts off, by early_identify_cpu(). So move the init_cpu_info()
call out of identify_cpu() into its callers, and skip it for the boot
CPU on 64-bit.

Secondary CPUs call init_cpu_info() then identify_cpu() from
identify_secondary_cpu(), with interrupts off, exactly as
before. 32-bit still resets in identify_boot_cpu(): it relies on the
reset for the no-CPUID cpuid_level default.

[1] https://lore.kernel.org/bpf/20260610175651.647515-1-ihor.solodrai@linux.dev/

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 arch/x86/kernel/cpu/common.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 8f093c6c7ccc..cd3d7c37c174 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1986,8 +1986,6 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 
 	c->loops_per_jiffy = loops_per_jiffy;
 
-	init_cpu_info(c);
-
 	if (!cpuid_feature())
 		identify_cpu_without_cpuid(c);
 
@@ -2156,6 +2154,13 @@ static void identify_cpu_32(struct cpuinfo_x86 *c)
 
 static __init void identify_boot_cpu(void)
 {
+	/*
+	 * The boot CPU's capabilities were already scanned in early_identify_cpu().
+	 * However 32-bit still needs to init_cpu_info() here for the no-CPUID
+	 * cpuid_level default.
+	 */
+	if (IS_ENABLED(CONFIG_X86_32))
+		init_cpu_info(&boot_cpu_data);
 	identify_cpu(&boot_cpu_data);
 	if (HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT))
 		pr_info("CET detected: Indirect Branch Tracking enabled\n");
@@ -2178,6 +2183,7 @@ void identify_secondary_cpu(unsigned int cpu)
 		*c = boot_cpu_data;
 	c->cpu_index = cpu;
 
+	init_cpu_info(c);
 	identify_cpu(c);
 	identify_cpu_32(c);
 	x86_spec_ctrl_setup_ap();
-- 
2.54.0