arm64: alternatives: have callbacks take a cap

Today, callback alternatives are special-cased within
__apply_alternatives(), and are applied alongside patching for system
capabilities as ARM64_NCAPS is not part of the boot_capabilities feature
mask.

This special-casing is less than ideal. Giving special meaning to
ARM64_NCAPS for this requires some structures and loops to use
ARM64_NCAPS + 1 (AKA ARM64_NPATCHABLE), while others use ARM64_NCAPS.
It's also not immediately clear callback alternatives are only applied
when applying alternatives for system-wide features.

To make this a bit clearer, changes the way that callback alternatives
are identified to remove the special-casing of ARM64_NCAPS, and to allow
callback alternatives to be associated with a cpucap as with all other
alternatives.

New cpucaps, ARM64_ALWAYS_BOOT and ARM64_ALWAYS_SYSTEM are added which
are always detected alongside boot cpu capabilities and system
capabilities respectively. All existing callback alternatives are made
to use ARM64_ALWAYS_SYSTEM, and so will be patched at the same point
during the boot flow as before.

Subsequent patches will make more use of these new cpucaps.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20220912162210.3626215-7-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 14 insertions, 12 deletions

diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
index 2e18c9c0f612..9a071a5fcb67 100644
--- a/arch/arm64/kernel/alternative.c
+++ b/arch/arm64/kernel/alternative.c
@@ -21,6 +21,9 @@
 #define ALT_ORIG_PTR(a)		__ALT_PTR(a, orig_offset)
 #define ALT_REPL_PTR(a)		__ALT_PTR(a, alt_offset)
 
+#define ALT_CAP(a)		((a)->cpufeature & ~ARM64_CB_BIT)
+#define ALT_HAS_CB(a)		((a)->cpufeature & ARM64_CB_BIT)
+
 /* Volatile, as we may be patching the guts of READ_ONCE() */
 static volatile int all_alternatives_applied;
 
@@ -143,16 +146,15 @@ static void __nocfi __apply_alternatives(const struct alt_region *region,
 
 	for (alt = region->begin; alt < region->end; alt++) {
 		int nr_inst;
+		int cap = ALT_CAP(alt);
 
-		if (!test_bit(alt->cpufeature, feature_mask))
+		if (!test_bit(cap, feature_mask))
 			continue;
 
-		/* Use ARM64_CB_PATCH as an unconditional patch */
-		if (alt->cpufeature < ARM64_CB_PATCH &&
-		    !cpus_have_cap(alt->cpufeature))
+		if (!cpus_have_cap(cap))
 			continue;
 
-		if (alt->cpufeature == ARM64_CB_PATCH)
+		if (ALT_HAS_CB(alt))
 			BUG_ON(alt->alt_len != 0);
 		else
 			BUG_ON(alt->alt_len != alt->orig_len);
@@ -161,10 +163,10 @@ static void __nocfi __apply_alternatives(const struct alt_region *region,
 		updptr = is_module ? origptr : lm_alias(origptr);
 		nr_inst = alt->orig_len / AARCH64_INSN_SIZE;
 
-		if (alt->cpufeature < ARM64_CB_PATCH)
-			alt_cb = patch_alternative;
-		else
+		if (ALT_HAS_CB(alt))
 			alt_cb  = ALT_REPL_PTR(alt);
+		else
+			alt_cb = patch_alternative;
 
 		alt_cb(alt, origptr, updptr, nr_inst);
 
@@ -208,10 +210,10 @@ static int __apply_alternatives_multi_stop(void *unused)
 			cpu_relax();
 		isb();
 	} else {
-		DECLARE_BITMAP(remaining_capabilities, ARM64_NPATCHABLE);
+		DECLARE_BITMAP(remaining_capabilities, ARM64_NCAPS);
 
 		bitmap_complement(remaining_capabilities, boot_capabilities,
-				  ARM64_NPATCHABLE);
+				  ARM64_NCAPS);
 
 		BUG_ON(all_alternatives_applied);
 		__apply_alternatives(&kernel_alternatives, false,
@@ -254,9 +256,9 @@ void apply_alternatives_module(void *start, size_t length)
 		.begin	= start,
 		.end	= start + length,
 	};
-	DECLARE_BITMAP(all_capabilities, ARM64_NPATCHABLE);
+	DECLARE_BITMAP(all_capabilities, ARM64_NCAPS);
 
-	bitmap_fill(all_capabilities, ARM64_NPATCHABLE);
+	bitmap_fill(all_capabilities, ARM64_NCAPS);
 
 	__apply_alternatives(&region, true, &all_capabilities[0]);
 }