arm64/entry: deduplicate SW PAN entry/exit routines

Factor the 12 copies of the SW PAN entry and exit code into callable
subroutines, and use alternatives patching to either emit a 'bl'
instruction to call them, or a NOP if h/w PAN is found to be available
at runtime.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20200721083315.4816-1-ardb@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/entry.S

@@ -209,28 +209,9 @@ alternative_cb_end
 	add	x29, sp, #S_STACKFRAME
 
 #ifdef CONFIG_ARM64_SW_TTBR0_PAN
-	/*
-	 * Set the TTBR0 PAN bit in SPSR. When the exception is taken from
-	 * EL0, there is no need to check the state of TTBR0_EL1 since
-	 * accesses are always enabled.
-	 * Note that the meaning of this bit differs from the ARMv8.1 PAN
-	 * feature as all TTBR0_EL1 accesses are disabled, not just those to
-	 * user mappings.
-	 */
-alternative_if ARM64_HAS_PAN
-	b	1f				// skip TTBR0 PAN
+alternative_if_not ARM64_HAS_PAN
+	bl	__swpan_entry_el\el
 alternative_else_nop_endif
-
-	.if	\el != 0
-	mrs	x21, ttbr0_el1
-	tst	x21, #TTBR_ASID_MASK		// Check for the reserved ASID
-	orr	x23, x23, #PSR_PAN_BIT		// Set the emulated PAN in the saved SPSR
-	b.eq	1f				// TTBR0 access already disabled
-	and	x23, x23, #~PSR_PAN_BIT		// Clear the emulated PAN in the saved SPSR
-	.endif
-
-	__uaccess_ttbr0_disable x21
-1:
 #endif
 
 	stp	x22, x23, [sp, #S_PC]
@@ -284,34 +265,9 @@ alternative_else_nop_endif
 	.endif
 
 #ifdef CONFIG_ARM64_SW_TTBR0_PAN
-	/*
-	 * Restore access to TTBR0_EL1. If returning to EL0, no need for SPSR
-	 * PAN bit checking.
-	 */
-alternative_if ARM64_HAS_PAN
-	b	2f				// skip TTBR0 PAN
+alternative_if_not ARM64_HAS_PAN
+	bl	__swpan_exit_el\el
 alternative_else_nop_endif
-
-	.if	\el != 0
-	tbnz	x22, #22, 1f			// Skip re-enabling TTBR0 access if the PSR_PAN_BIT is set
-	.endif
-
-	__uaccess_ttbr0_enable x0, x1
-
-	.if	\el == 0
-	/*
-	 * Enable errata workarounds only if returning to user. The only
-	 * workaround currently required for TTBR0_EL1 changes are for the
-	 * Cavium erratum 27456 (broadcast TLBI instructions may cause I-cache
-	 * corruption).
-	 */
-	bl	post_ttbr_update_workaround
-	.endif
-1:
-	.if	\el != 0
-	and	x22, x22, #~PSR_PAN_BIT		// ARMv8.0 CPUs do not understand this bit
-	.endif
-2:
 #endif
 
 	.if	\el == 0
@@ -391,6 +347,49 @@ alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0
 	sb
 	.endm
 
+#ifdef CONFIG_ARM64_SW_TTBR0_PAN
+	/*
+	 * Set the TTBR0 PAN bit in SPSR. When the exception is taken from
+	 * EL0, there is no need to check the state of TTBR0_EL1 since
+	 * accesses are always enabled.
+	 * Note that the meaning of this bit differs from the ARMv8.1 PAN
+	 * feature as all TTBR0_EL1 accesses are disabled, not just those to
+	 * user mappings.
+	 */
+SYM_CODE_START_LOCAL(__swpan_entry_el1)
+	mrs	x21, ttbr0_el1
+	tst	x21, #TTBR_ASID_MASK		// Check for the reserved ASID
+	orr	x23, x23, #PSR_PAN_BIT		// Set the emulated PAN in the saved SPSR
+	b.eq	1f				// TTBR0 access already disabled
+	and	x23, x23, #~PSR_PAN_BIT		// Clear the emulated PAN in the saved SPSR
+SYM_INNER_LABEL(__swpan_entry_el0, SYM_L_LOCAL)
+	__uaccess_ttbr0_disable x21
+1:	ret
+SYM_CODE_END(__swpan_entry_el1)
+
+	/*
+	 * Restore access to TTBR0_EL1. If returning to EL0, no need for SPSR
+	 * PAN bit checking.
+	 */
+SYM_CODE_START_LOCAL(__swpan_exit_el1)
+	tbnz	x22, #22, 1f			// Skip re-enabling TTBR0 access if the PSR_PAN_BIT is set
+	__uaccess_ttbr0_enable x0, x1
+1:	and	x22, x22, #~PSR_PAN_BIT		// ARMv8.0 CPUs do not understand this bit
+	ret
+SYM_CODE_END(__swpan_exit_el1)
+
+SYM_CODE_START_LOCAL(__swpan_exit_el0)
+	__uaccess_ttbr0_enable x0, x1
+	/*
+	 * Enable errata workarounds only if returning to user. The only
+	 * workaround currently required for TTBR0_EL1 changes are for the
+	 * Cavium erratum 27456 (broadcast TLBI instructions may cause I-cache
+	 * corruption).
+	 */
+	b	post_ttbr_update_workaround
+SYM_CODE_END(__swpan_exit_el0)
+#endif
+
 	.macro	irq_stack_entry
 	mov	x19, sp			// preserve the original sp
 #ifdef CONFIG_SHADOW_CALL_STACK