arm64: Check DAIF (and PMR) at task-switch time

When __switch_to() switches from a 'prev' task to a 'next' task, various
pieces of CPU state are expected to have specific values, such that
these do not need to be saved/restored. If any of these hold an
unexpected value when switching away from the prev task, they could lead
to surprising behaviour in the context of the next task, and it would be
difficult to determine where they were configured to their unexpected
value.

Add some checks for DAIF and PMR at task-switch time so that we can
detect such issues.

Signed-off-by: Mark Rutland <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Jinjie Ruan <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vladimir Murzin <[email protected]>
Cc: Will Deacon <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>

Author: mrutland-arm

arch/arm64/kernel/process.c

@@ -699,6 +699,29 @@ void update_sctlr_el1(u64 sctlr)
 	isb();
 }
 
+static inline void debug_switch_state(void)
+{
+	if (system_uses_irq_prio_masking()) {
+		unsigned long daif_expected = 0;
+		unsigned long daif_actual = read_sysreg(daif);
+		unsigned long pmr_expected = GIC_PRIO_IRQOFF;
+		unsigned long pmr_actual = read_sysreg_s(SYS_ICC_PMR_EL1);
+
+		WARN_ONCE(daif_actual != daif_expected ||
+			  pmr_actual != pmr_expected,
+			  "Unexpected DAIF + PMR: 0x%lx + 0x%lx (expected 0x%lx + 0x%lx)\n",
+			  daif_actual, pmr_actual,
+			  daif_expected, pmr_expected);
+	} else {
+		unsigned long daif_expected = DAIF_PROCCTX_NOIRQ;
+		unsigned long daif_actual = read_sysreg(daif);
+
+		WARN_ONCE(daif_actual != daif_expected,
+			  "Unexpected DAIF value: 0x%lx (expected 0x%lx)\n",
+			  daif_actual, daif_expected);
+	}
+}
+
 /*
  * Thread switching.
  */
@@ -708,6 +731,8 @@ struct task_struct *__switch_to(struct task_struct *prev,
 {
 	struct task_struct *last;
 
+	debug_switch_state();
+
 	fpsimd_thread_switch(next);
 	tls_thread_switch(next);
 	hw_breakpoint_thread_switch(next);