[RFC PATCH 5/6] arm64: Detect current view of GIC priorities

@@ -188,6 +188,11 @@ Before jumping into the kernel, the following conditions must be met:
   the kernel image will be entered must be initialised by software at a
   higher exception level to prevent execution in an UNKNOWN state.

+  - SCR_EL3.FIQ must have the same value across all CPUs the kernel is
+    executing on.
+  - The value of SCR_EL3.FIQ must be the same as the one present at boot
+    time whenever the kernel is executing.
+
   For systems with a GICv3 interrupt controller to be used in v3 mode:
   - If EL3 is present:
     ICC_SRE_EL3.Enable (bit 3) must be initialiased to 0b1.

@@ -124,6 +124,11 @@ static inline void gic_write_bpr1(u32 val)
 	write_sysreg_s(val, SYS_ICC_BPR1_EL1);
 }

+static inline u32 gic_read_rpr(void)
+{
+	return read_sysreg_s(SYS_ICC_RPR_EL1);
+}
+
 #define gic_read_typer(c)		readq_relaxed(c)
 #define gic_write_irouter(v, c)		writeq_relaxed(v, c)
 #define gic_read_lpir(c)		readq_relaxed(c)

@@ -217,6 +217,12 @@ static inline int arch_irqs_disabled_flags(unsigned long flags)
 		!(ARCH_FLAGS_GET_PMR(flags) & ICC_PMR_EL1_EN_BIT);
 }

+/* Mask IRQs at CPU level instead of GIC level */
+static inline void arch_irqs_daif_disable(void)
+{
+	asm volatile ("msr daifset, #2" : : : "memory");
+}
+
 void maybe_switch_to_sysreg_gic_cpuif(void);

 #endif /* CONFIG_IRQFLAGS_GIC_MASKING */

@@ -304,6 +304,7 @@
 #define SYS_ICC_SRE_EL1			sys_reg(3, 0, 12, 12, 5)
 #define SYS_ICC_IGRPEN0_EL1		sys_reg(3, 0, 12, 12, 6)
 #define SYS_ICC_IGRPEN1_EL1		sys_reg(3, 0, 12, 12, 7)
+#define SYS_ICC_RPR_EL1			sys_reg(3, 0, 12, 11, 3)

 #define SYS_CONTEXTIDR_EL1		sys_reg(3, 0, 13, 0, 1)
 #define SYS_TPIDR_EL1			sys_reg(3, 0, 13, 0, 4)

@@ -63,6 +63,10 @@ struct gic_chip_data {
 static struct gic_chip_data gic_data __read_mostly;
 static struct static_key supports_deactivate = STATIC_KEY_INIT_TRUE;

+#ifdef CONFIG_USE_ICC_SYSREGS_FOR_IRQFLAGS
+DEFINE_STATIC_KEY_FALSE(have_non_secure_prio_view);
+#endif
+
 static struct gic_kvm_info gic_v3_kvm_info;
 static DEFINE_PER_CPU(bool, has_rss);

@@ -997,6 +1001,84 @@ static int partition_domain_translate(struct irq_domain *d,
 	.select = gic_irq_domain_select,
 };

+#ifdef CONFIG_USE_ICC_SYSREGS_FOR_IRQFLAGS
+/*
+ * The behaviours of RPR and PMR registers differ depending on the value of
+ * SCR_EL3.FIQ, while the behaviour of priority registers of the distributor
+ * and redistributors is always the same.
+ *
+ * If SCR_EL3.FIQ == 1, the values used for RPR and PMR are the same as the ones
+ * programmed in the distributor and redistributors registers.
+ *
+ * Otherwise, the value presented by RPR as well as the value which will be
+ * compared against PMR is: (GIC_(R)DIST_PRI[irq] >> 1) | 0x80;
+ *
+ * see GICv3/GICv4 Architecture Specification (IHI0069D):
+ * - section 4.8.1 Non-secure accesses to register fields for Secure interrupt
+ *   priorities.
+ * - Figure 4-7 Secure read of the priority field for a Non-secure Group 1
+ *   interrupt.
+ */
+static void __init gic_detect_prio_view(void)
+{
+	/*
+	 * Randomly picked SGI, must be <= 8 as other SGIs might be
+	 * used by the firmware.
+	 */
+	const u32 fake_irqnr = 7;
+	const u32 fake_irqmask = BIT(fake_irqnr);
+	void __iomem * const rdist_base = gic_data_rdist_sgi_base();
+	unsigned long irq_flags;
+	u32 acked_irqnr;
+	bool was_enabled;
+
+	irq_flags = arch_local_save_flags();
+
+	arch_irqs_daif_disable();
+
+	was_enabled = (readl_relaxed(rdist_base + GICD_ISENABLER) &
+		       fake_irqmask);
+
+	if (!was_enabled)
+		writel_relaxed(fake_irqmask, rdist_base + GICD_ISENABLER);
+
+	/* Need to unmask to acknowledge the IRQ */
+	gic_write_pmr(ICC_PMR_EL1_UNMASKED);
+	dsb(sy);
+
+	/* Fake a pending SGI */
+	writel_relaxed(fake_irqmask, rdist_base + GICD_ISPENDR);
+	dsb(sy);
+
+	do {
+		acked_irqnr = gic_read_iar();
+
+		if (acked_irqnr == fake_irqnr) {
+			if (gic_read_rpr() == gic_get_irq_prio(acked_irqnr,
+							       rdist_base))
+				static_branch_enable(&have_non_secure_prio_view);
+		} else {
+			pr_warn("Unexpected IRQ for priority detection: %u\n",
+				acked_irqnr);
+		}
+
+		if (acked_irqnr < 1020) {
+			gic_write_eoir(acked_irqnr);
+			if (static_key_true(&supports_deactivate))
+				gic_write_dir(acked_irqnr);
+		}
+	} while (acked_irqnr == ICC_IAR1_EL1_SPURIOUS);
+
+	/* Restore enabled state */
+	if (!was_enabled) {
+		writel_relaxed(fake_irqmask, rdist_base + GICD_ICENABLER);
+		gic_redist_wait_for_rwp();
+	}
+
+	arch_local_irq_restore(irq_flags);
+}
+#endif
+
 static int __init gic_init_bases(void __iomem *dist_base,
 				 struct redist_region *rdist_regs,
 				 u32 nr_redist_regions,

@@ -1057,6 +1139,10 @@ static int __init gic_init_bases(void __iomem *dist_base,
 	gic_cpu_init();
 	gic_cpu_pm_init();

+#ifdef CONFIG_USE_ICC_SYSREGS_FOR_IRQFLAGS
+	gic_detect_prio_view();
+#endif
+
 	return 0;

 out_free:
--