[lldb][AArch64] Invalidate SVG prior to reconfiguring ZA regdef

[DavidSpickett]

This fixes a bug where writing vg during streaming mode
could prevent you reading za directly afterwards.

vg is invalidated just prior to us reading it in AArch64Reconfigure,
but svg was not. This lead to some situations where vg would be
updated or cleared and re-read, but svg would not be.

This meant it had some undefined value which lead to errors
that prevented us reading ZA. Likely we received a lot more
data than we were expecting.

There are at least 2 ways to get into this situation:

• Explicit write by the user to vg.
• We have just stopped and need to get the potentially new svg and vg.

The first is handled by invalidating svg client side before fetching the new one. This also
covers some but not all of the second scenario. For the second, I've made writes to vg
invalidate svg by noting this in the register information.

Whichever one of those kicks in, we'll get the latest value of svg.

The bug may depend on timing, I could not find a consistent way
to trigger it. I originally found it when checking whether za
is disabled after a vg change, so I've added checks for that
to TestZAThreadedDynamic.

The SVE VG version of the bug did show up on the buildbot,
but not consistently. So it's possible that TestZAThreadedDynamic
does in fact cover this, but I haven't run it enough times to know.

[DavidSpickett]

Replaces https://reviews.llvm.org/D158514.

[llvmbot]

@llvm/pr-subscribers-lldb

Changes

This fixes a bug where writing vg during streaming mode could prevent you reading za directly afterwards.

vg is invalided when the write happens. SVE is reconfigured by reading back the new value from the remote. So far so good.

For SME, we read SVG which in some situations appeared to remain valid but have a value of 0 (perhaps it had never been read). This meant lldb expected the size of za to be 0 bytes and rejected the remote's response.

To fix this, invalidate svg before reconfiguring. This ensures that the value used is the latest one from the remote.

The bug may depend on timing, I could not find a consistent way to trigger it. I originally found it when checking whether za is disabled after a vg change, so I've added checks for that to TestZAThreadedDynamic.

Differential Revision: https://reviews.llvm.org/D158514

---

Full diff: https://github.com/llvm/llvm-project/pull/66768.diff

2 Files Affected:

• (modified) lldb/source/Plugins/Process/gdb-remote/GDBRemoteRegisterContext.cpp (+5)
• (modified) lldb/test/API/commands/register/register/aarch64_za_register/za_dynamic_resize/TestZAThreadedDynamic.py (+2)

diff --git a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteRegisterContext.cpp b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteRegisterContext.cpp
index 16dde28f6ee5396..13365dc312f5b66 100644
--- a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteRegisterContext.cpp
+++ b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteRegisterContext.cpp
@@ -781,6 +781,11 @@ void GDBRemoteRegisterContext::AArch64Reconfigure() {
   std::optional<uint64_t> svg_reg_value;
   const RegisterInfo *svg_reg_info = m_reg_info_sp->GetRegisterInfo("svg");
   if (svg_reg_info) {
+    // When vg is written it is automatically made invalid. Writing vg will also
+    // change svg if we're in streaming mode but it will not be made invalid
+    // so do this manually so the following read gets the latest svg value.
+    SetRegisterIsValid(svg_reg_info, false);
+
     uint32_t svg_reg_num = svg_reg_info->kinds[eRegisterKindLLDB];
     uint64_t reg_value = ReadRegisterAsUnsigned(svg_reg_num, fail_value);
     if (reg_value != fail_value && reg_value <= 32)
diff --git a/lldb/test/API/commands/register/register/aarch64_za_register/za_dynamic_resize/TestZAThreadedDynamic.py b/lldb/test/API/commands/register/register/aarch64_za_register/za_dynamic_resize/TestZAThreadedDynamic.py
index 65d1071c26b2a34..d2a26ce71bde1d8 100644
--- a/lldb/test/API/commands/register/register/aarch64_za_register/za_dynamic_resize/TestZAThreadedDynamic.py
+++ b/lldb/test/API/commands/register/register/aarch64_za_register/za_dynamic_resize/TestZAThreadedDynamic.py
@@ -125,11 +125,13 @@ def za_test_impl(self, enable_za):
                 self.runCmd("thread select %d" % (idx + 1))
                 self.check_za_register(4, 2)
                 self.runCmd("register write vg 2")
+                self.check_disabled_za_register(2)
 
             elif stopped_at_line_number == thY_break_line1:
                 self.runCmd("thread select %d" % (idx + 1))
                 self.check_za_register(2, 3)
                 self.runCmd("register write vg 4")
+                self.check_disabled_za_register(4)
 
         self.runCmd("thread continue 2")
         self.runCmd("thread continue 3")

[DavidSpickett]

I've just pushed a similar fix for SVE's VG (fdc265b). Fixes some test flakiness so I've pushed it directly.

[DavidSpickett]

ping!

[DavidSpickett]

ping!

I know you're very busy at the moment, but if you have time to review this week this is the most important patch I've got queued.