On Tue, Apr 01, 2025 at 12:28:33PM -0700, Nathan Chancellor wrote:

Hi Charlie,

On Wed, Mar 19, 2025 at 11:35:20AM -0700, Charlie Jenkins wrote:

quoted

Implement the runtime constant infrastructure for riscv. Use this
infrastructure to generate constants to be used by the d_hash()
function.

This is the riscv variant of commit 94a2bc0f611c ("arm64: add 'runtime
constant' support") and commit e3c92e81711d ("runtime constants: add
x86 architecture support").

Signed-off-by: Charlie Jenkins <redacted>
Reviewed-by: Alexandre Ghiti <redacted>

...

quoted

diff --git a/arch/riscv/include/asm/runtime-const.h b/arch/riscv/include/asm/runtime-const.h

...

quoted

+#define RISCV_RUNTIME_CONST_64_ZBA				\
+	".option push\n\t"					\
+	".option arch,+zba\n\t"					\
+	"slli	%[__tmp],%[__tmp],32\n\t"			\
+	"add.uw %[__ret],%[__ret],%[__tmp]\n\t"			\
+	"nop\n\t"						\
+	"nop\n\t"						\
+	".option pop\n\t"					\

...

quoted

+#if defined(CONFIG_RISCV_ISA_ZBA) && defined(CONFIG_RISCV_ISA_ZBKB)

...

quoted

+#elif defined(CONFIG_RISCV_ISA_ZBA)
+#define runtime_const_ptr(sym)						\
+({									\
+	typeof(sym) __ret, __tmp;					\
+	asm_inline(RISCV_RUNTIME_CONST_64_PREAMBLE			\
+		ALTERNATIVE(						\
+			RISCV_RUNTIME_CONST_64_BASE,			\
+			RISCV_RUNTIME_CONST_64_ZBA,			\
+			0, RISCV_ISA_EXT_ZBA, 1				\
+		)							\
+		RISCV_RUNTIME_CONST_64_POSTAMBLE(sym)			\
+		: [__ret] "=r" (__ret), [__tmp] "=r" (__tmp));		\
+	__ret;								\
+})

This breaks the build for clang versions 16 and earlier because they do
not support '.option arch' and it is used in CONFIG_RISCV_ISA_ZBA, which
has no dependencies and it is default on.

  $ make -skj"$(nproc)" ARCH=riscv LLVM=1 mrproper defconfig fs/dcache.o
  fs/dcache.c:117:9: warning: unknown option, expected 'push', 'pop', 'rvc', 'norvc', 'relax' or 'norelax' [-Winline-asm]
          return runtime_const_ptr(dentry_hashtable) +
                 ^
  arch/riscv/include/asm/runtime-const.h:103:4: note: expanded from macro 'runtime_const_ptr'
                          RISCV_RUNTIME_CONST_64_ZBA,                     \
                          ^
  arch/riscv/include/asm/runtime-const.h:57:17: note: expanded from macro 'RISCV_RUNTIME_CONST_64_ZBA'
          ".option push\n\t"                                      \
                         ^
  <inline asm>:32:10: note: instantiated into assembly here
          .option arch,+zba
                  ^
  fs/dcache.c:117:9: error: instruction requires the following: 'Zba' (Address Generation Instructions)
          return runtime_const_ptr(dentry_hashtable) +
                 ^
  arch/riscv/include/asm/runtime-const.h:103:4: note: expanded from macro 'runtime_const_ptr'
                          RISCV_RUNTIME_CONST_64_ZBA,                     \
                          ^
  arch/riscv/include/asm/runtime-const.h:59:30: note: expanded from macro 'RISCV_RUNTIME_CONST_64_ZBA'
          "slli   %[__tmp],%[__tmp],32\n\t"                       \
                                        ^
  <inline asm>:34:2: note: instantiated into assembly here
          add.uw a2,a2,a3
          ^
  ...

  $ rg 'OPTION_ARCH|ZBA' .config
  364:CONFIG_RISCV_ISA_ZBA=y

Should it grow a dependency on AS_HAS_OPTION_ARCH or should there be a
different fix?

This should have been fixed by Alex's patch [1]. Zba is in an awkward
state because BPF generates Zba code without the need for toolchain
support.

[1] https://lore.kernel.org/all/20250328115422.253670-1-alexghiti@rivosinc.com/ (local)

Cheers,
Nathan