[llvm] [llvm-exegesis][AArch64] Check for PAC keys before disabling them (PR #138643)

[David Green via llvm-commits]

davemgreen wrote:

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Subject: Re: [llvm/llvm-project] [llvm-exegesis][AArch64] Check for PAC keys before disabling them (PR #138643)


@davemgreen commented on this pull request.

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In llvm/tools/llvm-exegesis/lib/AArch64/Target.cpp<https://github.com/llvm/llvm-project/pull/138643#discussion_r2084674919>:

> @@ -207,15 +213,38 @@ class ExegesisAArch64Target : public ExegesisTarget {

     if (isPointerAuth(Opcode)) {
 #if defined(__aarch64__) && defined(__linux__)
+
+      // For some systems with existing PAC keys set, it is better to
+      // check the existing state of the key before setting it.
+      // For systems without PAC, this is a No-op but with PAC, it is
+      // safer to check the existing key state and then disable/enable them.
+      // Hence the guard for switching.
+      errno = 0;
+      unsigned long PacKeys = prctl_wrapper(PR_PAC_GET_ENABLED_KEYS,
+                                            0,  // unused
+                                            0,  // unused
+                                            0,  // unused
+                                            0); // unused
+      if ((long)PacKeys < 0) {
+        if (errno == EINVAL) {
+          return "PAuth not supported on this system";


I have a system where pacbti is not enabled at the system level (the linux distro does not support it yet), but it is fine to check the latency/throughput of the instructions - they are just already disabled. Trying this patch leads to a "AUTDA: PAuth not supported on this system" but we might as well allow people to try and test the instructions if they are available.

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https://github.com/llvm/llvm-project/pull/138643