arm64: errata: Fix KVM Spectre-v2 mitigation selection for Cortex-A57/A72

Both the Spectre-v2 and Spectre-BHB mitigations involve running a sequence
immediately after exiting a guest, before any branches. In the stable
kernels these sequences are built by copying templates into an empty vector
slot.

For Spectre-BHB, Cortex-A57 and A72 require the branchy loop with k=8.
If Spectre-v2 needs mitigating at the same time, a firmware call to EL3 is
needed. The work EL3 does at this point is also enough to mitigate
Spectre-BHB.

When enabling the Spectre-BHB mitigation, spectre_bhb_enable_mitigation()
should check if a slot has already been allocated for Spectre-v2, meaning
no work is needed for Spectre-BHB.

This check was missed in the earlier backport, add it.

Fixes: 4dd8aae585 ("arm64: Mitigate spectre style branch history side channels")
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arm64/kernel/cpu_errata.c

@@ -876,7 +876,13 @@ void spectre_bhb_enable_mitigation(const struct arm64_cpu_capabilities *entry)
 	} else if (spectre_bhb_loop_affected(SCOPE_LOCAL_CPU)) {
 		switch (spectre_bhb_loop_affected(SCOPE_SYSTEM)) {
 		case 8:
-			kvm_setup_bhb_slot(__spectre_bhb_loop_k8_start);
+			/*
+			 * A57/A72-r0 will already have selected the
+			 * spectre-indirect vector, which is sufficient
+			 * for BHB too.
+			 */
+			if (!__this_cpu_read(bp_hardening_data.fn))
+				kvm_setup_bhb_slot(__spectre_bhb_loop_k8_start);
 			break;
 		case 24:
 			kvm_setup_bhb_slot(__spectre_bhb_loop_k24_start);