KVM: arm64: nVHE: Don't consume host SErrors with ESB

The ESB at the start of the host vector may cause SErrors to be consumed
to DISR_EL1. However, this is not checked for the host so the SError
could go unhandled.

Remove the ESB so that SErrors are not consumed but are instead left
pending for the host to consume. __guest_enter already defers entry into
a guest if there are any SErrors pending.
Signed-off-by: Andrew Scull <ascull@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: James Morse <james.morse@arm.com>
Link: https://lore.kernel.org/r/20200915104643.2543892-8-ascull@google.com

arch/arm64/kvm/hyp/nvhe/host.S

@@ -25,7 +25,6 @@ SYM_FUNC_END(__hyp_do_panic)
 .macro host_el1_sync_vect
 	.align 7
 .L__vect_start\@:
-	esb
 	stp	x0, x1, [sp, #-16]!
 	mrs	x0, esr_el2
 	lsr	x0, x0, #ESR_ELx_EC_SHIFT
@@ -80,6 +79,11 @@ SYM_FUNC_END(__hyp_do_panic)
 .endm
 
 /*
+ * The host vector does not use an ESB instruction in order to avoid consuming
+ * SErrors that should only be consumed by the host. Guest entry is deferred by
+ * __guest_enter if there are any pending asynchronous exceptions so hyp will
+ * always return to the host without having consumerd host SErrors.
+ *
  * CONFIG_KVM_INDIRECT_VECTORS is not applied to the host vectors because the
  * host knows about the EL2 vectors already, and there is no point in hiding
  * them.