ARM: smp: defer TPIDRURO update for SMP v6 configurations too

Defer TPIDURO updates for user space until exit also for CPU_V6+SMP
configurations so that we can decide at runtime whether to use it to
carry the current pointer, provided that we are running on a CPU that
actually implements this register. This is needed for
THREAD_INFO_IN_TASK support for UP systems, which requires that all SMP
capable systems use the TPIDRURO based access to 'current' as the only
remaining alternative will be a global variable which only works on UP.
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Marc Zyngier <maz@kernel.org>
Tested-by: Vladimir Murzin <vladimir.murzin@arm.com> # ARMv7M

arch/arm/include/asm/tls.h

@@ -18,13 +18,14 @@
 	.endm
 
 	.macro switch_tls_v6, base, tp, tpuser, tmp1, tmp2
-	ldr	\tmp1, =elf_hwcap
-	ldr	\tmp1, [\tmp1, #0]
+	ldr_va	\tmp1, elf_hwcap
 	mov	\tmp2, #0xffff0fff
 	tst	\tmp1, #HWCAP_TLS		@ hardware TLS available?
 	streq	\tp, [\tmp2, #-15]		@ set TLS value at 0xffff0ff0
 	mrcne	p15, 0, \tmp2, c13, c0, 2	@ get the user r/w register
+#ifndef CONFIG_SMP
 	mcrne	p15, 0, \tp, c13, c0, 3		@ yes, set TLS register
+#endif
 	mcrne	p15, 0, \tpuser, c13, c0, 2	@ set user r/w register
 	strne	\tmp2, [\base, #TI_TP_VALUE + 4] @ save it
 	.endm
@@ -43,7 +44,7 @@
 #elif defined(CONFIG_CPU_V6)
 #define tls_emu		0
 #define has_tls_reg		(elf_hwcap & HWCAP_TLS)
-#define defer_tls_reg_update	0
+#define defer_tls_reg_update	IS_ENABLED(CONFIG_SMP)
 #define switch_tls	switch_tls_v6
 #elif defined(CONFIG_CPU_32v6K)
 #define tls_emu		0
@@ -81,11 +82,11 @@ static inline void set_tls(unsigned long val)
 	 */
 	barrier();
 
-	if (!tls_emu && !defer_tls_reg_update) {
-		if (has_tls_reg) {
+	if (!tls_emu) {
+		if (has_tls_reg && !defer_tls_reg_update) {
 			asm("mcr p15, 0, %0, c13, c0, 3"
 			    : : "r" (val));
-		} else {
+		} else if (!has_tls_reg) {
 #ifdef CONFIG_KUSER_HELPERS
 			/*
 			 * User space must never try to access this

arch/arm/kernel/entry-header.S

@@ -292,12 +292,21 @@
 
 
 	.macro	restore_user_regs, fast = 0, offset = 0
-#if defined(CONFIG_CPU_32v6K) && !defined(CONFIG_CPU_V6)
+#if defined(CONFIG_CPU_32v6K) || defined(CONFIG_SMP)
+#if defined(CONFIG_CPU_V6) && defined(CONFIG_SMP)
+ALT_SMP(b	.L1_\@	)
+ALT_UP( nop		)
+	ldr_va	r1, elf_hwcap
+	tst	r1, #HWCAP_TLS			@ hardware TLS available?
+	beq	.L2_\@
+.L1_\@:
+#endif
 	@ The TLS register update is deferred until return to user space so we
 	@ can use it for other things while running in the kernel
 	get_thread_info r1
 	ldr	r1, [r1, #TI_TP_VALUE]
 	mcr	p15, 0, r1, c13, c0, 3		@ set TLS register
+.L2_\@:
 #endif
 
 	uaccess_enable r1, isb=0