arm64: add ARM64_HAS_GIC_PRIO_RELAXED_SYNC cpucap

When Priority Mask Hint Enable (PMHE) == 0b1, the GIC may use the PMR
value to determine whether to signal an IRQ to a PE, and consequently
after a change to the PMR value, a DSB SY may be required to ensure that
interrupts are signalled to a CPU in finite time. When PMHE == 0b0,
interrupts are always signalled to the relevant PE, and all masking
occurs locally, without requiring a DSB SY.

Since commit:

  f226650 ("arm64: Relax ICC_PMR_EL1 accesses when ICC_CTLR_EL1.PMHE is clear")

... we handle this dynamically: in most cases a static key is used to
determine whether to issue a DSB SY, but the entry code must read from
ICC_CTLR_EL1 as static keys aren't accessible from plain assembly.

It would be much nicer to use an alternative instruction sequence for
the DSB, as this would avoid the need to read from ICC_CTLR_EL1 in the
entry code, and for most other code this will result in simpler code
generation with fewer instructions and fewer branches.

This patch adds a new ARM64_HAS_GIC_PRIO_RELAXED_SYNC cpucap which is
only set when ICC_CTLR_EL1.PMHE == 0b0 (and GIC priority masking is in
use). This allows us to replace the existing users of the
`gic_pmr_sync` static key with alternative sequences which default to a
DSB SY and are relaxed to a NOP when PMHE is not in use.

The entry assembly management of the PMR is slightly restructured to use
a branch (rather than multiple NOPs) when priority masking is not in
use. This is more in keeping with other alternatives in the entry
assembly, and permits the use of a separate alternatives for the
PMHE-dependent DSB SY (and removal of the conditional branch this
currently requires). For consistency I've adjusted both the save and
restore paths.

According to bloat-o-meter, when building defconfig +
CONFIG_ARM64_PSEUDO_NMI=y this shrinks the kernel text by ~4KiB:

| add/remove: 4/2 grow/shrink: 42/310 up/down: 332/-5032 (-4700)

The resulting vmlinux is ~66KiB smaller, though the resulting Image size
is unchanged due to padding and alignment:

| [mark@lakrids:~/src/linux]% ls -al vmlinux-*
| -rwxr-xr-x 1 mark mark 137508344 Jan 17 14:11 vmlinux-after
| -rwxr-xr-x 1 mark mark 137575440 Jan 17 13:49 vmlinux-before
| [mark@lakrids:~/src/linux]% ls -al Image-*
| -rw-r--r-- 1 mark mark 38777344 Jan 17 14:11 Image-after
| -rw-r--r-- 1 mark mark 38777344 Jan 17 13:49 Image-before

Prior to this patch we did not verify the state of ICC_CTLR_EL1.PMHE on
secondary CPUs. As of this patch this is verified by the cpufeature code
when using GIC priority masking (i.e. when using pseudo-NMIs).

Note that since commit:

  7e3a57f ("arm64: Document ICC_CTLR_EL3.PMHE setting requirements")

... Documentation/arm64/booting.rst specifies:

|      - ICC_CTLR_EL3.PMHE (bit 6) must be set to the same value across
|        all CPUs the kernel is executing on, and must stay constant
|        for the lifetime of the kernel.

... so that should not adversely affect any compliant systems, and as
we'll only check for the absense of PMHE when using pseudo-NMIs, this
will only fire when such mismatch will adversely affect the system.

Signed-off-by: Mark Rutland <[email protected]>
Reviewed-by: Marc Zyngier <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: Will Deacon <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>

Author: mrutland-arm

arch/arm/include/asm/arch_gicv3.h

@@ -252,5 +252,10 @@ static inline void gic_arch_enable_irqs(void)
 	WARN_ON_ONCE(true);
 }
 
+static inline bool gic_has_relaxed_pmr_sync(void)
+{
+	return false;
+}
+
 #endif /* !__ASSEMBLY__ */
 #endif /* !__ASM_ARCH_GICV3_H */

arch/arm64/include/asm/arch_gicv3.h

@@ -190,5 +190,10 @@ static inline void gic_arch_enable_irqs(void)
 	asm volatile ("msr daifclr, #3" : : : "memory");
 }
 
+static inline bool gic_has_relaxed_pmr_sync(void)
+{
+	return cpus_have_cap(ARM64_HAS_GIC_PRIO_RELAXED_SYNC);
+}
+
 #endif /* __ASSEMBLY__ */
 #endif /* __ASM_ARCH_GICV3_H */

arch/arm64/include/asm/barrier.h

@@ -11,6 +11,8 @@
 
 #include <linux/kasan-checks.h>
 
+#include <asm/alternative-macros.h>
+
 #define __nops(n)	".rept	" #n "\nnop\n.endr\n"
 #define nops(n)		asm volatile(__nops(n))
 
@@ -41,10 +43,11 @@
 #ifdef CONFIG_ARM64_PSEUDO_NMI
 #define pmr_sync()						\
 	do {							\
-		extern struct static_key_false gic_pmr_sync;	\
-								\
-		if (static_branch_unlikely(&gic_pmr_sync))	\
-			dsb(sy);				\
+		asm volatile(					\
+		ALTERNATIVE_CB("dsb sy",			\
+			       ARM64_HAS_GIC_PRIO_RELAXED_SYNC,	\
+			       alt_cb_patch_nops)		\
+		);						\
 	} while(0)
 #else
 #define pmr_sync()	do {} while (0)

arch/arm64/kernel/cpufeature.c

@@ -2056,6 +2056,34 @@ static bool can_use_gic_priorities(const struct arm64_cpu_capabilities *entry,
 
 	return enable_pseudo_nmi;
 }
+
+static bool has_gic_prio_relaxed_sync(const struct arm64_cpu_capabilities *entry,
+				      int scope)
+{
+	/*
+	 * If we're not using priority masking then we won't be poking PMR_EL1,
+	 * and there's no need to relax synchronization of writes to it, and
+	 * ICC_CTLR_EL1 might not be accessible and we must avoid reads from
+	 * that.
+	 *
+	 * ARM64_HAS_GIC_PRIO_MASKING has a lower index, and is a boot CPU
+	 * feature, so will be detected earlier.
+	 */
+	BUILD_BUG_ON(ARM64_HAS_GIC_PRIO_RELAXED_SYNC <= ARM64_HAS_GIC_PRIO_MASKING);
+	if (!cpus_have_cap(ARM64_HAS_GIC_PRIO_MASKING))
+		return false;
+
+	/*
+	 * When Priority Mask Hint Enable (PMHE) == 0b0, PMR is not used as a
+	 * hint for interrupt distribution, a DSB is not necessary when
+	 * unmasking IRQs via PMR, and we can relax the barrier to a NOP.
+	 *
+	 * Linux itself doesn't use 1:N distribution, so has no need to
+	 * set PMHE. The only reason to have it set is if EL3 requires it
+	 * (and we can't change it).
+	 */
+	return (gic_read_ctlr() & ICC_CTLR_EL1_PMHE_MASK) == 0;
+}
 #endif
 
 #ifdef CONFIG_ARM64_BTI
@@ -2546,6 +2574,14 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.type = ARM64_CPUCAP_STRICT_BOOT_CPU_FEATURE,
 		.matches = can_use_gic_priorities,
 	},
+	{
+		/*
+		 * Depends on ARM64_HAS_GIC_PRIO_MASKING
+		 */
+		.capability = ARM64_HAS_GIC_PRIO_RELAXED_SYNC,
+		.type = ARM64_CPUCAP_STRICT_BOOT_CPU_FEATURE,
+		.matches = has_gic_prio_relaxed_sync,
+	},
 #endif
 #ifdef CONFIG_ARM64_E0PD
 	{

arch/arm64/kernel/entry.S

@@ -311,13 +311,16 @@ alternative_else_nop_endif
 	.endif
 
 #ifdef CONFIG_ARM64_PSEUDO_NMI
-	/* Save pmr */
-alternative_if ARM64_HAS_GIC_PRIO_MASKING
+alternative_if_not ARM64_HAS_GIC_PRIO_MASKING
+	b	.Lskip_pmr_save\@
+alternative_else_nop_endif
+
 	mrs_s	x20, SYS_ICC_PMR_EL1
 	str	x20, [sp, #S_PMR_SAVE]
 	mov	x20, #GIC_PRIO_IRQON | GIC_PRIO_PSR_I_SET
 	msr_s	SYS_ICC_PMR_EL1, x20
-alternative_else_nop_endif
+
+.Lskip_pmr_save\@:
 #endif
 
 	/*
@@ -336,15 +339,19 @@ alternative_else_nop_endif
 	.endif
 
 #ifdef CONFIG_ARM64_PSEUDO_NMI
-	/* Restore pmr */
-alternative_if ARM64_HAS_GIC_PRIO_MASKING
+alternative_if_not ARM64_HAS_GIC_PRIO_MASKING
+	b	.Lskip_pmr_restore\@
+alternative_else_nop_endif
+
 	ldr	x20, [sp, #S_PMR_SAVE]
 	msr_s	SYS_ICC_PMR_EL1, x20
-	mrs_s	x21, SYS_ICC_CTLR_EL1
-	tbz	x21, #6, .L__skip_pmr_sync\@	// Check for ICC_CTLR_EL1.PMHE
-	dsb	sy				// Ensure priority change is seen by redistributor
-.L__skip_pmr_sync\@:
+
+	/* Ensure priority change is seen by redistributor */
+alternative_if_not ARM64_HAS_GIC_PRIO_RELAXED_SYNC
+	dsb	sy
 alternative_else_nop_endif
+
+.Lskip_pmr_restore\@:
 #endif
 
 	ldp	x21, x22, [sp, #S_PC]		// load ELR, SPSR

arch/arm64/kernel/image-vars.h

@@ -67,9 +67,7 @@ KVM_NVHE_ALIAS(__hyp_stub_vectors);
 KVM_NVHE_ALIAS(vgic_v2_cpuif_trap);
 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

arch/arm64/tools/cpucaps

@@ -30,6 +30,7 @@ HAS_GENERIC_AUTH_ARCH_QARMA5
 HAS_GENERIC_AUTH_IMP_DEF
 HAS_GIC_CPUIF_SYSREGS
 HAS_GIC_PRIO_MASKING
+HAS_GIC_PRIO_RELAXED_SYNC
 HAS_LDAPR
 HAS_LSE_ATOMICS
 HAS_NO_FPSIMD

drivers/irqchip/irq-gic-v3.c

@@ -89,15 +89,6 @@ static DEFINE_STATIC_KEY_TRUE(supports_deactivate_key);
  */
 static DEFINE_STATIC_KEY_FALSE(supports_pseudo_nmis);
 
-/*
- * Global static key controlling whether an update to PMR allowing more
- * interrupts requires to be propagated to the redistributor (DSB SY).
- * And this needs to be exported for modules to be able to enable
- * interrupts...
- */
-DEFINE_STATIC_KEY_FALSE(gic_pmr_sync);
-EXPORT_SYMBOL(gic_pmr_sync);
-
 DEFINE_STATIC_KEY_FALSE(gic_nonsecure_priorities);
 EXPORT_SYMBOL(gic_nonsecure_priorities);
 
@@ -1768,16 +1759,8 @@ static void gic_enable_nmi_support(void)
 	for (i = 0; i < gic_data.ppi_nr; i++)
 		refcount_set(&ppi_nmi_refs[i], 0);
 
-	/*
-	 * Linux itself doesn't use 1:N distribution, so has no need to
-	 * set PMHE. The only reason to have it set is if EL3 requires it
-	 * (and we can't change it).
-	 */
-	if (gic_read_ctlr() & ICC_CTLR_EL1_PMHE_MASK)
-		static_branch_enable(&gic_pmr_sync);
-
 	pr_info("Pseudo-NMIs enabled using %s ICC_PMR_EL1 synchronisation\n",
-		static_branch_unlikely(&gic_pmr_sync) ? "forced" : "relaxed");
+		gic_has_relaxed_pmr_sync() ? "relaxed" : "forced");
 
 	/*
 	 * How priority values are used by the GIC depends on two things: