arm64: Add per-cpu infrastructure to call ARCH_WORKAROUND_2

In a heterogeneous system, we can end up with both affected and
unaffected CPUs. Let's check their status before calling into the
firmware.
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/cpu_errata.c

@@ -233,6 +233,8 @@ enable_smccc_arch_workaround_1(const struct arm64_cpu_capabilities *entry)
 #endif	/* CONFIG_HARDEN_BRANCH_PREDICTOR */
 
 #ifdef CONFIG_ARM64_SSBD
+DEFINE_PER_CPU_READ_MOSTLY(u64, arm64_ssbd_callback_required);
+
 void __init arm64_update_smccc_conduit(struct alt_instr *alt,
 				       __le32 *origptr, __le32 *updptr,
 				       int nr_inst)

arch/arm64/kernel/entry.S

@@ -140,8 +140,10 @@ alternative_else_nop_endif
 
 	// This macro corrupts x0-x3. It is the caller's duty
 	// to save/restore them if required.
-	.macro	apply_ssbd, state
+	.macro	apply_ssbd, state, targ, tmp1, tmp2
 #ifdef CONFIG_ARM64_SSBD
+	ldr_this_cpu	\tmp2, arm64_ssbd_callback_required, \tmp1
+	cbz	\tmp2, \targ
 	mov	w0, #ARM_SMCCC_ARCH_WORKAROUND_2
 	mov	w1, #\state
 alternative_cb	arm64_update_smccc_conduit
@@ -176,12 +178,13 @@ alternative_cb_end
 	ldr	x19, [tsk, #TSK_TI_FLAGS]	// since we can unmask debug
 	disable_step_tsk x19, x20		// exceptions when scheduling.
 
-	apply_ssbd 1
+	apply_ssbd 1, 1f, x22, x23
 
 #ifdef CONFIG_ARM64_SSBD
 	ldp	x0, x1, [sp, #16 * 0]
 	ldp	x2, x3, [sp, #16 * 1]
 #endif
+1:
 
 	mov	x29, xzr			// fp pointed to user-space
 	.else
@@ -323,8 +326,8 @@ alternative_if ARM64_WORKAROUND_845719
 alternative_else_nop_endif
 #endif
 3:
-	apply_ssbd 0
-
+	apply_ssbd 0, 5f, x0, x1
+5:
 	.endif
 
 	msr	elr_el1, x21			// set up the return data