arm64: avoid double ISB on kernel entry

Although an ISB is required in order to make the MTE-related system
register update to GCR_EL1 effective, and the same is true for
PAC-related updates to SCTLR_EL1 or APIAKey{Hi,Lo}_EL1, we issue two
ISBs on machines that support both features while we only need to
issue one. To avoid the unnecessary additional ISB, remove the ISBs
from the PAC and MTE-specific alternative blocks and add a couple
of additional blocks that cause us to only execute one ISB if both
features are supported.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/Idee7e8114d5ae5a0b171d06220a0eb4bb015a51c
Link: https://lore.kernel.org/r/20210727205439.2557419-1-pcc@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/entry.S

@@ -184,7 +184,6 @@ alternative_else_nop_endif
 	ldr_l	\tmp, gcr_kernel_excl
 
 	mte_set_gcr \tmp, \tmp2
-	isb
 1:
 #endif
 	.endm
@@ -257,7 +256,6 @@ alternative_if ARM64_HAS_ADDRESS_AUTH
 	orr	x0, x0, SCTLR_ELx_ENIA
 	msr	sctlr_el1, x0
 2:
-	isb
 alternative_else_nop_endif
 #endif
 
@@ -265,6 +263,19 @@ alternative_else_nop_endif
 
 	mte_set_kernel_gcr x22, x23
 
+	/*
+	 * Any non-self-synchronizing system register updates required for
+	 * kernel entry should be placed before this point.
+	 */
+alternative_if ARM64_MTE
+	isb
+	b	1f
+alternative_else_nop_endif
+alternative_if ARM64_HAS_ADDRESS_AUTH
+	isb
+alternative_else_nop_endif
+1:
+
 	scs_load tsk
 	.else
 	add	x21, sp, #PT_REGS_SIZE