[RFC PATCH 4/5] sparc32: advertise the compare-and-swap trap in AT_HWCAP

[Magnus Lindholm]

A process that wants the kernel to perform a compare-and-swap has no way
to find out whether this kernel will do it, and issuing the trap to find
out is not an option: on a kernel without it the trap is a bad trap and
the process dies.

Guessing is worse than dying. On sparc64 ta 0x11 is not an illegal
instruction but the old 64-bit system call trap, so a process that issued
it there would make a wild syscall rather than take a signal.

Advertise it in AT_HWCAP so the question can be asked first.  Bit
0x10000000 is unused on sparc32 and on sparc64, so it reads as clear in
exactly the cases where the trap is absent: an older kernel, or sparc64
compat mode, where a 32-bit process reads its hwcap word from elf_64.h.
Reserve the same bit there with a comment so it is not handed to something
else later and mistaken for this.

A caller that finds the bit clear must fall back to whatever it can do by
itself or report the operation unsupported. A private lock is not a
general answer: it cannot make a word atomic against another process,
which is the reason for the trap in the first place.

Signed-off-by: Magnus Lindholm <linmag7@xxxxxxxxx>
---
 arch/sparc/include/asm/elf_32.h | 7 ++++++-
 arch/sparc/include/asm/elf_64.h | 3 +++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/sparc/include/asm/elf_32.h b/arch/sparc/include/asm/elf_32.h
index 37a6016c9ccd..c60d803cf556 100644
--- a/arch/sparc/include/asm/elf_32.h
+++ b/arch/sparc/include/asm/elf_32.h
@@ -64,6 +64,10 @@
 #define HWCAP_SPARC_MULDIV      8
 #define HWCAP_SPARC_V9		16
 #define HWCAP_SPARC_ULTRA3	32
+/* Kernel compare-and-swap trap available; see
+ * Documentation/arch/sparc/cas-trap.rst.  Do not issue the trap if clear.
+ */
+#define HWCAP_SPARC_CASTRAP	0x10000000
 
 #define CORE_DUMP_USE_REGSET
 
@@ -121,7 +125,8 @@ typedef struct {
 
 /* Most sun4m's have them all.  */
 #define ELF_HWCAP	(HWCAP_SPARC_FLUSH | HWCAP_SPARC_STBAR | \
-			 HWCAP_SPARC_SWAP | HWCAP_SPARC_MULDIV)
+			 HWCAP_SPARC_SWAP | HWCAP_SPARC_MULDIV | \
+			 HWCAP_SPARC_CASTRAP)
 
 /* This yields a string that ld.so will use to load implementation
    specific libraries for optimization.  This is more specific in
diff --git a/arch/sparc/include/asm/elf_64.h b/arch/sparc/include/asm/elf_64.h
index 694ed081cf8d..a2117a81b6d8 100644
--- a/arch/sparc/include/asm/elf_64.h
+++ b/arch/sparc/include/asm/elf_64.h
@@ -98,6 +98,9 @@
  */
 #define HWCAP_SPARC_CRYPTO	0x04000000 /* CRYPTO insns available */
 #define HWCAP_SPARC_ADI		0x08000000 /* ADI available */
+/* 0x10000000 is HWCAP_SPARC_CASTRAP on sparc32 and must stay clear here;
+ * a 32-bit process reads this word too and there is no such trap on sparc64.
+ */
 
 #define CORE_DUMP_USE_REGSET
 
-- 
2.43.0