powerpc/64s: Remove the "fast endian switch" syscall

The non-standard "fast endian switch" syscall was added in 2008[1],
but was never widely used. It was disabled by default in 2017[2], and
there's no evidence it's ever been used since.

Remove it entirely.

A normal endian switch syscall was added in 2015[3].

[1]: 745a14c ("[POWERPC] Add fast little-endian switch system call")
[2]: 529d235 ("powerpc: Add a proper syscall for switching endianness")
[3]: 727f136 ("powerpc: Disable the fast-endian switch syscall by default")

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240823070830.1269033-1-mpe@ellerman.id.au

Author: mpe

arch/powerpc/Kconfig.debug

@@ -379,12 +379,6 @@ config FAIL_IOMMU
 
 	  If you are unsure, say N.
 
-config PPC_FAST_ENDIAN_SWITCH
-	bool "Deprecated fast endian-switch syscall"
-	depends on DEBUG_KERNEL && PPC_BOOK3S_64
-	help
-	  If you're unsure what this is, say N.
-
 config KASAN_SHADOW_OFFSET
 	hex
 	depends on KASAN

arch/powerpc/kernel/exceptions-64s.S

@@ -1989,13 +1989,6 @@ INT_DEFINE_END(system_call)
 	INTERRUPT_TO_KERNEL
 #endif
 
-#ifdef CONFIG_PPC_FAST_ENDIAN_SWITCH
-BEGIN_FTR_SECTION
-	cmpdi	r0,0x1ebe
-	beq-	1f
-END_FTR_SECTION_IFSET(CPU_FTR_REAL_LE)
-#endif
-
 	/* We reach here with PACA in r13, r13 in r9. */
 	mfspr	r11,SPRN_SRR0
 	mfspr	r12,SPRN_SRR1
@@ -2015,16 +2008,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_REAL_LE)
 	b	system_call_common
 #endif
 	.endif
-
-#ifdef CONFIG_PPC_FAST_ENDIAN_SWITCH
-	/* Fast LE/BE switch system call */
-1:	mfspr	r12,SPRN_SRR1
-	xori	r12,r12,MSR_LE
-	mtspr	SPRN_SRR1,r12
-	mr	r13,r9
-	RFI_TO_USER	/* return to userspace */
-	b	.	/* prevent speculative execution */
-#endif
 .endm
 
 EXC_REAL_BEGIN(system_call, 0xc00, 0x100)