irqchip/gic-v3: Reset BPR during initialization

Currently, when running on FVP, CPU 0 boots up with its BPR changed from
the reset value. This renders it impossible to (preemptively) prioritize
interrupts on CPU 0.

This is harmless on normal systems since Linux typically does not
support preemptive interrupts. It does however cause problems in
systems with additional changes (such as patches for NMI simulation).

Many thanks to Andrew Thoelke for suggesting the BPR as having the
potential to harm preemption.

Suggested-by: Andrew Thoelke <andrew.thoelke@arm.com>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

3 files changed, 20 insertions, 0 deletions

diff --git a/arch/arm/include/asm/arch_gicv3.h b/arch/arm/include/asm/arch_gicv3.h
index e08d15184056..dfe4002812da 100644
--- a/arch/arm/include/asm/arch_gicv3.h
+++ b/arch/arm/include/asm/arch_gicv3.h
@@ -34,6 +34,7 @@
 #define ICC_CTLR                        __ACCESS_CP15(c12, 0, c12, 4)
 #define ICC_SRE                         __ACCESS_CP15(c12, 0, c12, 5)
 #define ICC_IGRPEN1                     __ACCESS_CP15(c12, 0, c12, 7)
+#define ICC_BPR1                        __ACCESS_CP15(c12, 0, c12, 3)
 
 #define ICC_HSRE                        __ACCESS_CP15(c12, 4, c9, 5)
 
@@ -157,6 +158,11 @@ static inline void gic_write_sre(u32 val)
         isb();
 }
 
+static inline void gic_write_bpr1(u32 val)
+{
+        asm volatile("mcr " __stringify(ICC_BPR1) : : "r" (val));
+}
+
 /*
  * Even in 32bit systems that use LPAE, there is no guarantee that the I/O
  * interface provides true 64bit atomic accesses, so using strd/ldrd doesn't
diff --git a/arch/arm64/include/asm/arch_gicv3.h b/arch/arm64/include/asm/arch_gicv3.h
index 8ec88e5b290f..fc2a0cb47b2c 100644
--- a/arch/arm64/include/asm/arch_gicv3.h
+++ b/arch/arm64/include/asm/arch_gicv3.h
@@ -28,6 +28,7 @@
 #define ICC_CTLR_EL1                    sys_reg(3, 0, 12, 12, 4)
 #define ICC_SRE_EL1                     sys_reg(3, 0, 12, 12, 5)
 #define ICC_GRPEN1_EL1                  sys_reg(3, 0, 12, 12, 7)
+#define ICC_BPR1_EL1                    sys_reg(3, 0, 12, 12, 3)
 
 #define ICC_SRE_EL2                     sys_reg(3, 4, 12, 9, 5)
 
@@ -165,6 +166,11 @@ static inline void gic_write_sre(u32 val)
         isb();
 }
 
+static inline void gic_write_bpr1(u32 val)
+{
+        asm volatile("msr_s " __stringify(ICC_BPR1_EL1) ", %0" : : "r" (val));
+}
+
 #define gic_read_typer(c)               readq_relaxed(c)
 #define gic_write_irouter(v, c)         writeq_relaxed(v, c)
 
diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
index ede5672ab34d..ecc5b2360c7a 100644
--- a/drivers/irqchip/irq-gic-v3.c
+++ b/drivers/irqchip/irq-gic-v3.c
@@ -495,6 +495,14 @@ static void gic_cpu_sys_reg_init(void)
         /* Set priority mask register */
         gic_write_pmr(DEFAULT_PMR_VALUE);
 
+        /*
+         * Some firmwares hand over to the kernel with the BPR changed from
+         * its reset value (and with a value large enough to prevent
+         * any pre-emptive interrupts from working at all). Writing a zero
+         * to BPR restores is reset value.
+         */
+        gic_write_bpr1(0);
+
         if (static_key_true(&supports_deactivate)) {
                 /* EOI drops priority only (mode 1) */
                 gic_write_ctlr(ICC_CTLR_EL1_EOImode_drop);