[go] internal/runtime/sys: only use speculation barrier for DIT on Apple Silicon

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Roland Shoemaker (Gerrit)

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Feb 24, 2026, 4:59:02 PM (11 hours ago) Feb 24

to goph...@pubsubhelper.golang.org, golang-...@googlegroups.com, Go LUCI, Keith Randall, Keith Randall, golang-co...@googlegroups.com

Roland Shoemaker submitted the change with unreviewed changes

Unreviewed changes

1 is the latest approved patch-set.
The change was submitted with unreviewed changes in the following files:

```
The name of the file: src/internal/runtime/sys/dit_arm64.s
Insertions: 6, Deletions: 0.

@@ -12,6 +12,12 @@
     TBNZ $0, R1, ret
     MSR $1, DIT
 #ifdef GOOS_darwin
+    // Arm documents that barriers are not necessary when writing to, or reading
+    // from, PSTATE fields. However, Apple documentation indicates that barriers
+    // should be used, in particular when setting the PSTATE.DIT field. Barriers
+    // aren't cheap, so only use them on Apple silicon for now.
+    //
+    // See go.dev/issue/77776.
     MOVBU internal∕cpu·ARM64+const_offsetARM64HasSB(SB), R2
     TBZ $0, R2, sbFallback
     SB
```

Change information

Commit message:

internal/runtime/sys: only use speculation barrier for DIT on Apple Silicon

The Apple documentation [0] indicates that when enabling DIT (via MSR),
a speculation barrier should be used. The rationale for this is not
explained, but one could assume that they perhaps do not treat MSR of
a PSTATE bit as a context synchronization event, and thus the CPU might
speculatively execute instructions that would be affected by DIT before
the MSR takes effect.

Arm on the other hand [1], explicitly states that MSR of a PSTATE bit is
a context synchronization event, and thus speculation barriers are not
needed.

In order to square this circle, we keep the speculation barrier, but
only use it on Apple Silicon.

[0] http://go/appledoc/xcode/writing-arm64-code-for-apple-platforms#Enable-DIT-for-constant-time-cryptographic-operations
[1] https://developer.arm.com/documentation/100941/0101/Barriers?lang=en

Fixes #77776