Subject: [PATCH] ARM: vfp: use undef hook for VFP support detection

From: Ard Biesheuvel <[email-masked]>
Date: Fri, 18 Dec 2020 16:54:44 +0100

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.

Reported-by: "kernelci.org bot" <[email-masked]>
Signed-off-by: Ard Biesheuvel <[email-masked]>
---
 arch/arm/vfp/entry.S     | 17 -----------------
 arch/arm/vfp/vfpmodule.c | 23 +++++++++++++++++++----
 2 files changed, 19 insertions(+), 21 deletions(-)

diff --git a/arch/arm/vfp/entry.S b/arch/arm/vfp/entry.S
index 0186cf9da890..27b0a1f27fbd 100644
--- a/arch/arm/vfp/entry.S
+++ b/arch/arm/vfp/entry.S
@@ -37,20 +37,3 @@ ENDPROC(vfp_null_entry)
 	.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
diff --git a/arch/arm/vfp/vfpmodule.c b/arch/arm/vfp/vfpmodule.c
index c3b6451c18bd..285a1c7e7a3f 100644
--- a/arch/arm/vfp/vfpmodule.c
+++ b/arch/arm/vfp/vfpmodule.c
@@ -32,7 +32,6 @@
 /*
  * Our undef handlers (in entry.S)
  */
-asmlinkage void vfp_testing_entry(void);
 asmlinkage void vfp_support_entry(void);
 asmlinkage void vfp_null_entry(void);
 
@@ -43,7 +42,7 @@ asmlinkage void (*vfp_vector)(void) = vfp_null_entry;
  * Used in startup: set to non-zero if VFP checks fail
  * After startup, holds VFP architecture
  */
-unsigned int VFP_arch;
+static unsigned int VFP_arch;
 
 /*
  * The pointer to the vfpstate structure of the thread which currently
@@ -707,7 +706,7 @@ static int __init vfp_kmode_exception_hook_init(void)
 		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
@@ -753,6 +752,21 @@ EXPORT_SYMBOL(kernel_neon_end);
 
 #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.
  */
@@ -773,10 +787,11 @@ static int __init vfp_init(void)
 	 * 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: ");
-- 
2.17.1

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