ARM: 9044/1: vfp: use undef hook for VFP support detection

Commit f77ac2e378be9dd6 ("ARM: 9030/1: entry: omit FP emulation for UND
exceptions taken in kernel mode") failed to take into account that there
is in fact a case where we relied on this code path: during boot, the
VFP detection code issues a read of FPSID, which will trigger an undef
exception on cores that lack VFP support.

So let's reinstate this logic using an undef hook which is registered
only for the duration of the initcall to vpf_init(), and which sets
VFP_arch to a non-zero value - as before - if no VFP support is present.

Fixes: f77ac2e378be9dd6 ("ARM: 9030/1: entry: omit FP emulation for UND ...")
Reported-by: "kernelci.org bot" <[e-mail]>
Signed-off-by: Ard Biesheuvel <[e-mail]>
Signed-off-by: Russell King <[e-mail]>

arch/arm/vfp/entry.S

···
 	.align	2
 .LCvfp:
 	.word	vfp_vector
-
-@ This code is called if the VFP does not exist. It needs to flag the
-@ failure to the VFP initialisation code.
-
-	__INIT
-ENTRY(vfp_testing_entry)
-	dec_preempt_count_ti r10, r4
-	ldr	r0, VFP_arch_address
-	str	r0, [r0]		@ set to non-zero value
-	ret	r9			@ we have handled the fault
-ENDPROC(vfp_testing_entry)
-
-	.align	2
-VFP_arch_address:
-	.word	VFP_arch
-
-	__FINIT

arch/arm/vfp/vfpmodule.c

···
 /*
  * Our undef handlers (in entry.S)
  */
-asmlinkage void vfp_testing_entry(void);
 asmlinkage void vfp_support_entry(void);
 asmlinkage void vfp_null_entry(void);
 
···
  * Used in startup: set to non-zero if VFP checks fail
  * After startup, holds VFP architecture
  */
-unsigned int VFP_arch;
+static unsigned int __initdata VFP_arch;
 
 /*
  * The pointer to the vfpstate structure of the thread which currently
···
  * present on all CPUs within a SMP complex. Needs to be called prior to
  * vfp_init().
  */
-void vfp_disable(void)
+void __init vfp_disable(void)
 {
 	if (VFP_arch) {
 		pr_debug("%s: should be called prior to vfp_init\n", __func__);
···
 		register_undef_hook(&vfp_kmode_exception_hook[i]);
 	return 0;
 }
-core_initcall(vfp_kmode_exception_hook_init);
+subsys_initcall(vfp_kmode_exception_hook_init);
 
 /*
  * Kernel-side NEON support functions
···
 
 #endif /* CONFIG_KERNEL_MODE_NEON */
 
+static int __init vfp_detect(struct pt_regs *regs, unsigned int instr)
+{
+	VFP_arch = UINT_MAX;	/* mark as not present */
+	regs->ARM_pc += 4;
+	return 0;
+}
+
+static struct undef_hook vfp_detect_hook __initdata = {
+	.instr_mask	= 0x0c000e00,
+	.instr_val	= 0x0c000a00,
+	.cpsr_mask	= MODE_MASK,
+	.cpsr_val	= SVC_MODE,
+	.fn		= vfp_detect,
+};
+
 /*
  * VFP support code initialisation.
  */
···
 	 * The handler is already setup to just log calls, so
 	 * we just need to read the VFPSID register.
 	 */
-	vfp_vector = vfp_testing_entry;
+	register_undef_hook(&vfp_detect_hook);
 	barrier();
 	vfpsid = fmrx(FPSID);
 	barrier();
+	unregister_undef_hook(&vfp_detect_hook);
 	vfp_vector = vfp_null_entry;
 
 	pr_info("VFP support v0.3: ");