arm64: signal: Remove ISB when resetting POR_EL0

POR_EL0 is set to its most permissive value before setting up the
signal frame, to ensure that uaccess succeeds regardless of the
signal stack's pkey.

We are now tolerant to spurious POE faults. This means that we do
not strictly need to issue an ISB after updating POR_EL0, even when
followed by uaccess. The question is whether a fault is likely to
happen or not if the ISB is omitted; in this case the answer seems
to be no. If the regular stack is used, then it should already be
accessible. If the alternate signal stack is used, then a special
(inaccessible) pkey may be used - the assumption is that this
situation is very uncommon.

Remove the ISB to speed up the regular path - this should not have
any functional impact regardless of the scenario.

Signed-off-by: Kevin Brodsky <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>

Author: kevin-brodsky-arm

arch/arm64/kernel/signal.c

@@ -95,8 +95,11 @@ static void save_reset_user_access_state(struct user_access_state *ua_state)
 
 		ua_state->por_el0 = read_sysreg_s(SYS_POR_EL0);
 		write_sysreg_s(por_enable_all, SYS_POR_EL0);
-		/* Ensure that any subsequent uaccess observes the updated value */
-		isb();
+		/*
+		 * No ISB required as we can tolerate spurious Overlay faults -
+		 * the fault handler will check again based on the new value
+		 * of POR_EL0.
+		 */
 	}
 }