arm64: Remove logic to kill 32-bit tasks on 64-bit-only cores

The scheduler now knows enough about these braindead systems to place
32-bit tasks accordingly, so throw out the safety checks and allow the
ret-to-user path to avoid do_notify_resume() if there is nothing to do.
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20210730112443.23245-16-will@kernel.org

arch/arm64/kernel/process.c

@@ -469,15 +469,6 @@ static void erratum_1418040_thread_switch(struct task_struct *prev,
 	write_sysreg(val, cntkctl_el1);
 }
 
-static void compat_thread_switch(struct task_struct *next)
-{
-	if (!is_compat_thread(task_thread_info(next)))
-		return;
-
-	if (static_branch_unlikely(&arm64_mismatched_32bit_el0))
-		set_tsk_thread_flag(next, TIF_NOTIFY_RESUME);
-}
-
 static void update_sctlr_el1(u64 sctlr)
 {
 	/*
@@ -519,7 +510,6 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
 	ssbs_thread_switch(next);
 	erratum_1418040_thread_switch(prev, next);
 	ptrauth_thread_switch_user(next);
-	compat_thread_switch(next);
 
 	/*
 	 * Complete any pending TLB or cache maintenance on this CPU in case
@@ -621,10 +611,8 @@ void arch_setup_new_exec(void)
 		 * at the point of execve(), although we try a bit harder to
 		 * honour the cpuset hierarchy.
 		 */
-		if (static_branch_unlikely(&arm64_mismatched_32bit_el0)) {
+		if (static_branch_unlikely(&arm64_mismatched_32bit_el0))
 			force_compatible_cpus_allowed_ptr(current);
-			set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
-		}
 	} else if (static_branch_unlikely(&arm64_mismatched_32bit_el0)) {
 		relax_compatible_cpus_allowed_ptr(current);
 	}

arch/arm64/kernel/signal.c

@@ -912,19 +912,6 @@ static void do_signal(struct pt_regs *regs)
 	restore_saved_sigmask();
 }
 
-static bool cpu_affinity_invalid(struct pt_regs *regs)
-{
-	if (!compat_user_mode(regs))
-		return false;
-
-	/*
-	 * We're preemptible, but a reschedule will cause us to check the
-	 * affinity again.
-	 */
-	return !cpumask_test_cpu(raw_smp_processor_id(),
-				 system_32bit_el0_cpumask());
-}
-
 asmlinkage void do_notify_resume(struct pt_regs *regs,
 				 unsigned long thread_flags)
 {
@@ -952,19 +939,6 @@ asmlinkage void do_notify_resume(struct pt_regs *regs,
 			if (thread_flags & _TIF_NOTIFY_RESUME) {
 				tracehook_notify_resume(regs);
 				rseq_handle_notify_resume(NULL, regs);
-
-				/*
-				 * If we reschedule after checking the affinity
-				 * then we must ensure that TIF_NOTIFY_RESUME
-				 * is set so that we check the affinity again.
-				 * Since tracehook_notify_resume() clears the
-				 * flag, ensure that the compiler doesn't move
-				 * it after the affinity check.
-				 */
-				barrier();
-
-				if (cpu_affinity_invalid(regs))
-					force_sig(SIGKILL);
 			}
 
 			if (thread_flags & _TIF_FOREIGN_FPSTATE)