arm64: Introduce system_capabilities_finalized() marker

We finalize the system wide capabilities after the SMP CPUs
are booted by the kernel. This is used as a marker for deciding
various checks in the kernel. e.g, sanity check the hotplugged
CPUs for missing mandatory features.

However there is no explicit helper available for this in the
kernel. There is sys_caps_initialised, which is not exposed.
The other closest one we have is the jump_label arm64_const_caps_ready
which denotes that the capabilities are set and the capability checks
could use the individual jump_labels for fast path. This is
performed before setting the ELF Hwcaps, which must be checked
against the new CPUs. We also perform some of the other initialization
e.g, SVE setup, which is important for the use of FP/SIMD
where SVE is supported. Normally userspace doesn't get to run
before we finish this. However the in-kernel users may
potentially start using the neon mode. So, we need to
reject uses of neon mode before we are set. Instead of defining
a new marker for the completion of SVE setup, we could simply
reuse the arm64_const_caps_ready and enable it once we have
finished all the setup. Also we could expose this to the
various users as "system_capabilities_finalized()" to make
it more meaningful than "const_caps_ready".

Cc: Ard Biesheuvel <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Reviewed-by: Ard Biesheuvel <[e-mail]>
Reviewed-by: Catalin Marinas <[e-mail]>
Signed-off-by: Suzuki K Poulose <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/include/asm/cpufeature.h

···
 	       system_uses_irq_prio_masking();
 }
 
+static inline bool system_capabilities_finalized(void)
+{
+	return static_branch_likely(&arm64_const_caps_ready);
+}
+
 #define ARM64_BP_HARDEN_UNKNOWN		-1
 #define ARM64_BP_HARDEN_WA_NEEDED	0
 #define ARM64_BP_HARDEN_NOT_REQUIRED	1

arch/arm64/include/asm/kvm_host.h

···
 	 * wrong, and hyp will crash and burn when it uses any
 	 * cpus_have_const_cap() wrapper.
 	 */
-	BUG_ON(!static_branch_likely(&arm64_const_caps_ready));
+	BUG_ON(!system_capabilities_finalized());
 	__kvm_call_hyp((void *)pgd_ptr, hyp_stack_ptr, vector_ptr, tpidr_el2);
 
 	/*

arch/arm64/kernel/cpufeature.c

···
  * will be used to determine if a new booting CPU should
  * go through the verification process to make sure that it
  * supports the system capabilities, without using a hotplug
- * notifier.
+ * notifier. This is also used to decide if we could use
+ * the fast path for checking constant CPU caps.
  */
-static bool sys_caps_initialised;
-
-static inline void set_sys_caps_initialised(void)
+DEFINE_STATIC_KEY_FALSE(arm64_const_caps_ready);
+EXPORT_SYMBOL(arm64_const_caps_ready);
+static inline void finalize_system_capabilities(void)
 {
-	sys_caps_initialised = true;
+	static_branch_enable(&arm64_const_caps_ready);
 }
 
 static int dump_cpu_hwcaps(struct notifier_block *self, unsigned long v, void *p)
···
 
 		/* Probe vector lengths, unless we already gave up on SVE */
 		if (id_aa64pfr0_sve(read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1)) &&
-		    !sys_caps_initialised)
+		    !system_capabilities_finalized())
 			sve_update_vq_map();
 	}
 
···
 	 * Otherwise, this CPU should verify that it has all the system
 	 * advertised capabilities.
 	 */
-	if (!sys_caps_initialised)
+	if (!system_capabilities_finalized())
 		update_cpu_capabilities(SCOPE_LOCAL_CPU);
 	else
 		verify_local_cpu_capabilities();
···
 	update_cpu_capabilities(SCOPE_BOOT_CPU | SCOPE_LOCAL_CPU);
 	/* Enable the SCOPE_BOOT_CPU capabilities alone right away */
 	enable_cpu_capabilities(SCOPE_BOOT_CPU);
-}
-
-DEFINE_STATIC_KEY_FALSE(arm64_const_caps_ready);
-EXPORT_SYMBOL(arm64_const_caps_ready);
-
-static void __init mark_const_caps_ready(void)
-{
-	static_branch_enable(&arm64_const_caps_ready);
 }
 
 bool this_cpu_has_cap(unsigned int n)
···
 	u32 cwg;
 
 	setup_system_capabilities();
-	mark_const_caps_ready();
 	setup_elf_hwcaps(arm64_elf_hwcaps);
 
 	if (system_supports_32bit_el0())
···
 	minsigstksz_setup();
 
 	/* Advertise that we have computed the system capabilities */
-	set_sys_caps_initialised();
+	finalize_system_capabilities();
 
 	/*
 	 * Check for sane CTR_EL0.CWG value.

arch/arm64/kernel/process.c

···
 	 * Only allow a task to be preempted once cpufeatures have been
 	 * enabled.
 	 */
-	if (static_branch_likely(&arm64_const_caps_ready))
+	if (system_capabilities_finalized())
 		preempt_schedule_irq();
 }