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Subject[PATCH v3 2/2] irqchip/gic-v3: Support pseudo-NMIs when SCR_EL3.FIQ == 0
Date
The GIC's internal view of the priority mask register and the assigned
interrupt priorities are based on whether GIC security is enabled and
whether firmware routes Group 0 interrupts to EL3. At the moment, we
support priority masking when ICC_PMR_EL1 and interrupt priorities are
either both modified by the GIC, or both left unchanged.

Trusted Firmware-A's default interrupt routing model allows Group 0
interrupts to be delivered to the non-secure world (SCR_EL3.FIQ == 0).
Unfortunately, this is precisely the case that the GIC driver doesn't
support: ICC_PMR_EL1 remains unchanged, but the GIC's view of interrupt
priorities is different from the software programmed values.

Support pseudo-NMIs when SCR_EL3.FIQ == 0 by using a different value to
mask regular interrupts. All the other values remain the same.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
---
arch/arm64/include/asm/arch_gicv3.h | 8 ++++-
arch/arm64/include/asm/ptrace.h | 14 +++++++-
arch/arm64/kernel/image-vars.h | 2 ++
drivers/irqchip/irq-gic-v3.c | 56 ++++++++++++++++++++++-------
4 files changed, 66 insertions(+), 14 deletions(-)

diff --git a/arch/arm64/include/asm/arch_gicv3.h b/arch/arm64/include/asm/arch_gicv3.h
index 6647ae4f0231..880b9054d75c 100644
--- a/arch/arm64/include/asm/arch_gicv3.h
+++ b/arch/arm64/include/asm/arch_gicv3.h
@@ -153,7 +153,7 @@ static inline bool gic_prio_masking_enabled(void)

static inline void gic_pmr_mask_irqs(void)
{
- BUILD_BUG_ON(GICD_INT_DEF_PRI < (GIC_PRIO_IRQOFF |
+ BUILD_BUG_ON(GICD_INT_DEF_PRI < (__GIC_PRIO_IRQOFF |
GIC_PRIO_PSR_I_SET));
BUILD_BUG_ON(GICD_INT_DEF_PRI >= GIC_PRIO_IRQON);
/*
@@ -162,6 +162,12 @@ static inline void gic_pmr_mask_irqs(void)
* are applied to IRQ priorities
*/
BUILD_BUG_ON((0x80 | (GICD_INT_DEF_PRI >> 1)) >= GIC_PRIO_IRQON);
+ /*
+ * Same situation as above, but now we make sure that we can mask
+ * regular interrupts.
+ */
+ BUILD_BUG_ON((0x80 | (GICD_INT_DEF_PRI >> 1)) < (__GIC_PRIO_IRQOFF_NS |
+ GIC_PRIO_PSR_I_SET));
gic_write_pmr(GIC_PRIO_IRQOFF);
}

diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
index 966ed30ed5f7..997cf8c8cd52 100644
--- a/arch/arm64/include/asm/ptrace.h
+++ b/arch/arm64/include/asm/ptrace.h
@@ -31,9 +31,21 @@
* interrupt disabling temporarily does not rely on IRQ priorities.
*/
#define GIC_PRIO_IRQON 0xe0
-#define GIC_PRIO_IRQOFF (GIC_PRIO_IRQON & ~0x80)
+#define __GIC_PRIO_IRQOFF (GIC_PRIO_IRQON & ~0x80)
+#define __GIC_PRIO_IRQOFF_NS 0xa0
#define GIC_PRIO_PSR_I_SET (1 << 4)

+#define GIC_PRIO_IRQOFF \
+ ({ \
+ extern struct static_key_false gic_nonsecure_priorities;\
+ u8 __prio = __GIC_PRIO_IRQOFF; \
+ \
+ if (static_branch_unlikely(&gic_nonsecure_priorities)) \
+ __prio = __GIC_PRIO_IRQOFF_NS; \
+ \
+ __prio; \
+ })
+
/* Additional SPSR bits not exposed in the UABI */
#define PSR_MODE_THREAD_BIT (1 << 0)
#define PSR_IL_BIT (1 << 20)
diff --git a/arch/arm64/kernel/image-vars.h b/arch/arm64/kernel/image-vars.h
index 8982b68289b7..98a5215c1598 100644
--- a/arch/arm64/kernel/image-vars.h
+++ b/arch/arm64/kernel/image-vars.h
@@ -101,6 +101,8 @@ KVM_NVHE_ALIAS(vgic_v3_cpuif_trap);
/* Static key checked in pmr_sync(). */
#ifdef CONFIG_ARM64_PSEUDO_NMI
KVM_NVHE_ALIAS(gic_pmr_sync);
+/* Static key checked in GIC_PRIO_IRQOFF. */
+KVM_NVHE_ALIAS(gic_nonsecure_priorities);
#endif

/* EL2 exception handling */
diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
index aa9b43d412b2..bb55d89d7d49 100644
--- a/drivers/irqchip/irq-gic-v3.c
+++ b/drivers/irqchip/irq-gic-v3.c
@@ -75,16 +75,14 @@ static DEFINE_STATIC_KEY_TRUE(supports_deactivate_key);
*
* If SCR_EL3.FIQ == 1, the values writen to/read from PMR and RPR at non-secure
* EL1 are subject to a similar operation thus matching the priorities presented
- * from the (re)distributor when security is enabled.
+ * from the (re)distributor when security is enabled. When SCR_EL3.FIQ == 0,
+ * these values are unchanched by the GIC.
*
* 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.
- *
- * For now, we only support pseudo-NMIs if we have non-secure view of
- * priorities.
*/
static DEFINE_STATIC_KEY_FALSE(supports_pseudo_nmis);

@@ -97,6 +95,9 @@ static DEFINE_STATIC_KEY_FALSE(supports_pseudo_nmis);
DEFINE_STATIC_KEY_FALSE(gic_pmr_sync);
EXPORT_SYMBOL(gic_pmr_sync);

+DEFINE_STATIC_KEY_FALSE(gic_nonsecure_priorities);
+EXPORT_SYMBOL(gic_nonsecure_priorities);
+
/* ppi_nmi_refs[n] == number of cpus having ppi[n + 16] set as NMI */
static refcount_t *ppi_nmi_refs;

@@ -932,14 +933,20 @@ static void gic_cpu_sys_reg_init(void)
/* Set priority mask register */
if (!gic_prio_masking_enabled()) {
write_gicreg(DEFAULT_PMR_VALUE, ICC_PMR_EL1);
- } else {
+ } else if (gic_supports_nmi()) {
/*
* Mismatch configuration with boot CPU, the system is likely
* to die as interrupt masking will not work properly on all
* CPUs
+ *
+ * The boot CPU calls this function before enabling NMI support,
+ * and as a result we'll never see this warning in the boot path
+ * for that CPU.
*/
- WARN_ON(gic_supports_nmi() && group0 &&
- !gic_dist_security_disabled());
+ if (static_branch_unlikely(&gic_nonsecure_priorities))
+ WARN_ON(!group0 || gic_dist_security_disabled());
+ else
+ WARN_ON(group0 && !gic_dist_security_disabled());
}

/*
@@ -1544,11 +1551,6 @@ static void gic_enable_nmi_support(void)
if (!gic_prio_masking_enabled())
return;

- if (gic_has_group0() && !gic_dist_security_disabled()) {
- pr_warn("SCR_EL3.FIQ is cleared, cannot enable use of pseudo-NMIs\n");
- return;
- }
-
ppi_nmi_refs = kcalloc(gic_data.ppi_nr, sizeof(*ppi_nmi_refs), GFP_KERNEL);
if (!ppi_nmi_refs)
return;
@@ -1567,6 +1569,36 @@ static void gic_enable_nmi_support(void)
pr_info("Pseudo-NMIs enabled using %s ICC_PMR_EL1 synchronisation\n",
static_branch_unlikely(&gic_pmr_sync) ? "forced" : "relaxed");

+ /*
+ * How priority values are used by the GIC depends on two things:
+ * the security state of the GIC (controlled by the GICD_CTRL.DS bit)
+ * and if Group 0 interrupts can be delivered to Linux in the non-secure
+ * world as FIQs (controlled by the SCR_EL3.FIQ bit). These affect the
+ * the ICC_PMR_EL1 register and the priority that software assigns to
+ * interrupts:
+ *
+ * GICD_CTRL.DS | SCR_EL3.FIQ | ICC_PMR_EL1 | Group 1 priority
+ * -----------------------------------------------------------
+ * 1 | - | unchanged | unchanged
+ * -----------------------------------------------------------
+ * 0 | 1 | non-secure | non-secure
+ * -----------------------------------------------------------
+ * 0 | 0 | unchanged | non-secure
+ *
+ * where non-secure means that the value is right-shifted by one and the
+ * MSB bit set, to make it fit in the non-secure priority range.
+ *
+ * In the first two cases, where ICC_PMR_EL1 and the interrupt priority
+ * are both either modified or unchanged, we can use the same set of
+ * priorities.
+ *
+ * In the last case, where only the interrupt priorities are modified to
+ * be in the non-secure range, we use a different PMR value to mask IRQs
+ * and the rest of the values that we use remain unchanged.
+ */
+ if (gic_has_group0() && !gic_dist_security_disabled())
+ static_branch_enable(&gic_nonsecure_priorities);
+
static_branch_enable(&supports_pseudo_nmis);

if (static_branch_likely(&supports_deactivate_key))
--
2.28.0
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