[libunwind] Call __arm_za_disable before entering EH

[MacDue]

This is done by defining __arm_za_disable as a weak symbol with the assumption that if libunwind is being linked against SME-aware code, the ABI routines will be provided (by libgcc or compiler-rt).

[llvmbot]

@llvm/pr-subscribers-libunwind

Author: Benjamin Maxwell (MacDue)

Changes

This is done by defining __arm_za_disable as a weak symbol with the assumption that if libunwind is being linked against SME-aware code, the ABI routines will be provided (by libgcc or compiler-rt).

---

Full diff: https://github.com/llvm/llvm-project/pull/160905.diff

1 Files Affected:

• (modified) libunwind/src/UnwindLevel1.c (+27)

diff --git a/libunwind/src/UnwindLevel1.c b/libunwind/src/UnwindLevel1.c
index f3b451ad9b730..0a1d4be976830 100644
--- a/libunwind/src/UnwindLevel1.c
+++ b/libunwind/src/UnwindLevel1.c
@@ -186,6 +186,10 @@ unwind_phase1(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *except
 }
 extern int __unw_step_stage2(unw_cursor_t *);
 
+#if defined(__aarch64__)
+extern void __attribute__((weak)) __arm_za_disable(void);
+#endif
+
 #if defined(_LIBUNWIND_USE_GCS)
 // Enable the GCS target feature to permit gcspop instructions to be used.
 __attribute__((target("+gcs")))
@@ -198,6 +202,29 @@ unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor,
   _LIBUNWIND_TRACE_UNWINDING("unwind_phase2(ex_obj=%p)",
                              (void *)exception_object);
 
+#if defined(__aarch64__)
+  // The platform must ensure that all the following conditions are true on
+  // entry to EH:
+  //
+  // - PSTATE.SM is 0.
+  // - PSTATE.ZA is 0.
+  // - TPIDR2_EL0 is null.
+  //
+  // The first point is ensured by routines for throwing exceptions having a
+  // non-streaming interface. TPIDR2_EL0 is set to null and ZA disabled by
+  // calling __arm_za_disable.
+  //
+  // See:
+  // https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#exceptions
+  if (__arm_za_disable) {
+    // FIXME: Is SME is available and `__arm_za_disable` is not, this should
+    // abort.
+    __arm_za_disable();
+  } else {
+    _LIBUNWIND_DEBUG_LOG("failed to call __arm_za_disable in %s", __FUNCTION__);
+  }
+#endif
+
   // uc is initialized by __unw_getcontext in the parent frame. The first stack
   // frame walked is unwind_phase2.
   unsigned framesWalked = 1;

[sdesmalen-arm]

I'll leave it to code-owners/maintainers of libunwind to review or approve the PR further, but from an AArch64/SME point of view the change looks right to me.

[MacDue]

Kind ping @llvm/reviewers-libunwind, @MaskRay 🙂

[MacDue]

Does any libunwind reviewer have an opinion on this approach? If not, we plan to land this soon. An alternative approach would be to re-implement __arm_za_disable in libunwind (tested here: #165451), but it would be nice to avoid that.

(Ping @llvm/reviewers-libunwind)

[ojhunt]

This is mostly a style review - I don't really know what za is?

[ojhunt]

I think better would be gating on a feature specific flag - it makes it easier to understand whether something is written the way it because the feature is not supported, or because the implementation of that feature is different - in essence someone new comes along and says "there's an 'if !', does the mean it is not available, or the abi is different?"

[MacDue]

This should work on macOS too (as the same routines should be present); however, I think weak symbols are handled differently there (looks like it's weak_import instead). I'm going to try that on the CI (as I don't have a Mac to hand right now).

[ojhunt]

I believe we should link to something more stable like their tag versions, say https://github.com/ARM-software/abi-aa/blob/2025Q1/aapcs64/aapcs64.rst#exceptions

[MacDue]

I'm not sure what that'd be here? I'm aware GCC versions their symbols, but this symbol could be provided by libgcc/ibgcc_s or compiler-rt, and I'm not sure what version information is generally available.