arm64: unify asm-arch manipulation

Assemblers will reject instructions not supported by a target
architecture version, and so we must explicitly tell the assembler the
latest architecture version for which we want to assemble instructions
from.

We've added a few AS_HAS_ARMV8_<N> definitions for this, in addition to
an inconsistently named AS_HAS_PAC definition, from which arm64's
top-level Makefile determines the architecture version that we intend to
target, and generates the `asm-arch` variable.

To make this a bit clearer and easier to maintain, this patch reworks
the Makefile to determine asm-arch in a single if-else-endif chain.
AS_HAS_PAC, which is defined when the assembler supports
`-march=armv8.3-a`, is renamed to AS_HAS_ARMV8_3.

As the logic for armv8.3-a is lifted out of the block handling pointer
authentication, `asm-arch` may now be set to armv8.3-a regardless of
whether support for pointer authentication is selected. This means that
it will be possible to assemble armv8.3-a instructions even if we didn't
intend to, but this is consistent with our handling of other
architecture versions, and the compiler won't generate armv8.3-a
instructions regardless.

For the moment there's no need for an CONFIG_AS_HAS_ARMV8_1, as the code
for LSE atomics and LDAPR use individual `.arch_extension` entries and
do not require the baseline asm arch to be bumped to armv8.1-a. The
other armv8.1-a features (e.g. PAN) do not require assembler support.

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

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20230131105809.991288-2-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: mrutland-arm

arch/arm64/Kconfig

@@ -1818,7 +1818,7 @@ config ARM64_PTR_AUTH_KERNEL
 	bool "Use pointer authentication for kernel"
 	default y
 	depends on ARM64_PTR_AUTH
-	depends on (CC_HAS_SIGN_RETURN_ADDRESS || CC_HAS_BRANCH_PROT_PAC_RET) && AS_HAS_PAC
+	depends on (CC_HAS_SIGN_RETURN_ADDRESS || CC_HAS_BRANCH_PROT_PAC_RET) && AS_HAS_ARMV8_3
 	# Modern compilers insert a .note.gnu.property section note for PAC
 	# which is only understood by binutils starting with version 2.33.1.
 	depends on LD_IS_LLD || LD_VERSION >= 23301 || (CC_IS_GCC && GCC_VERSION < 90100)
@@ -1843,7 +1843,7 @@ config CC_HAS_SIGN_RETURN_ADDRESS
 	# GCC 7, 8
 	def_bool $(cc-option,-msign-return-address=all)
 
-config AS_HAS_PAC
+config AS_HAS_ARMV8_3
 	def_bool $(cc-option,-Wa$(comma)-march=armv8.3-a)
 
 config AS_HAS_CFI_NEGATE_RA_STATE

arch/arm64/Makefile

@@ -63,11 +63,6 @@ stack_protector_prepare: prepare0
 					include/generated/asm-offsets.h))
 endif
 
-ifeq ($(CONFIG_AS_HAS_ARMV8_2), y)
-# make sure to pass the newest target architecture to -march.
-asm-arch := armv8.2-a
-endif
-
 # Ensure that if the compiler supports branch protection we default it
 # off, this will be overridden if we are using branch protection.
 branch-prot-flags-y += $(call cc-option,-mbranch-protection=none)
@@ -88,25 +83,29 @@ branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET_BTI) := -mbranch-protectio
 else
 branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=$(PACRET-y)
 endif
-# -march=armv8.3-a enables the non-nops instructions for PAC, to avoid the
-# compiler to generate them and consequently to break the single image contract
-# we pass it only to the assembler. This option is utilized only in case of non
-# integrated assemblers.
-ifeq ($(CONFIG_AS_HAS_PAC), y)
-asm-arch := armv8.3-a
-endif
 endif
 
 KBUILD_CFLAGS += $(branch-prot-flags-y)
 
-ifeq ($(CONFIG_AS_HAS_ARMV8_4), y)
-# make sure to pass the newest target architecture to -march.
-asm-arch := armv8.4-a
-endif
-
+# Tell the assembler to support instructions from the latest target
+# architecture.
+#
+# For non-integrated assemblers we'll pass this on the command line, and for
+# integrated assemblers we'll define ARM64_ASM_ARCH and ARM64_ASM_PREAMBLE for
+# inline usage.
+#
+# We cannot pass the same arch flag to the compiler as this would allow it to
+# freely generate instructions which are not supported by earlier architecture
+# versions, which would prevent a single kernel image from working on earlier
+# hardware.
 ifeq ($(CONFIG_AS_HAS_ARMV8_5), y)
-# make sure to pass the newest target architecture to -march.
-asm-arch := armv8.5-a
+  asm-arch := armv8.5-a
+else ifeq ($(CONFIG_AS_HAS_ARMV8_4), y)
+  asm-arch := armv8.4-a
+else ifeq ($(CONFIG_AS_HAS_ARMV8_3), y)
+  asm-arch := armv8.3-a
+else ifeq ($(CONFIG_AS_HAS_ARMV8_2), y)
+  asm-arch := armv8.2-a
 endif
 
 ifdef asm-arch