arm64: entry: Make the kpti trampoline's kpti sequence optional

Spectre-BHB needs to add sequences to the vectors. Having one global
set of vectors is a problem for big/little systems where the sequence
is costly on cpus that are not vulnerable.

Making the vectors per-cpu in the style of KVM's bh_harden_hyp_vecs
requires the vectors to be generated by macros.

Make the kpti re-mapping of the kernel optional, so the macros can be
used without kpti.
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>

arch/arm64/kernel/entry.S

@@ -646,9 +646,10 @@ alternative_else_nop_endif
 	sub	\dst, \dst, PAGE_SIZE
 	.endm
 
-	.macro tramp_ventry, vector_start, regsize
+	.macro tramp_ventry, vector_start, regsize, kpti
 	.align	7
 1:
+	.if	\kpti == 1
 	.if	\regsize == 64
 	msr	tpidrro_el0, x30	// Restored in kernel_ventry
 	.endif
@@ -671,9 +672,14 @@ alternative_insn isb, nop, ARM64_WORKAROUND_QCOM_FALKOR_E1003
 alternative_if_not ARM64_WORKAROUND_CAVIUM_TX2_219_PRFM
 	prfm	plil1strm, [x30, #(1b - \vector_start)]
 alternative_else_nop_endif
+
 	msr	vbar_el1, x30
-	add	x30, x30, #(1b - \vector_start + 4)
 	isb
+	.else
+	ldr	x30, =vectors
+	.endif // \kpti == 1
+
+	add	x30, x30, #(1b - \vector_start + 4)
 	ret
 .org 1b + 128	// Did we overflow the ventry slot?
 	.endm
@@ -691,15 +697,15 @@ alternative_else_nop_endif
 	sb
 	.endm
 
-	.macro	generate_tramp_vector
+	.macro	generate_tramp_vector,	kpti
 .Lvector_start\@:
 	.space	0x400
 
 	.rept	4
-	tramp_ventry	.Lvector_start\@, 64
+	tramp_ventry	.Lvector_start\@, 64, \kpti
 	.endr
 	.rept	4
-	tramp_ventry	.Lvector_start\@, 32
+	tramp_ventry	.Lvector_start\@, 32, \kpti
 	.endr
 	.endm
 
@@ -710,7 +716,7 @@ alternative_else_nop_endif
 	.pushsection ".entry.tramp.text", "ax"
 	.align	11
 SYM_CODE_START_NOALIGN(tramp_vectors)
-	generate_tramp_vector
+	generate_tramp_vector	kpti=1
 SYM_CODE_END(tramp_vectors)
 
 SYM_CODE_START(tramp_exit_native)