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

[DavidSpickett]

Summary

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.

[DavidSpickett]

And if you're wondering, ZA being disabled after a vector length change is expected. lldb is just reporting what the kernel does here. lldb-server will still give you back a block of 0s though and that's what was not shown in this situation.

[DavidSpickett]

Rebase

[DavidSpickett]

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[DavidSpickett]

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[DavidSpickett]

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[DavidSpickett]

Moved to https://github.com/llvm/llvm-project/pull/66768.

lldb/source/Plugins/Process/gdb-remote/GDBRemoteRegisterContext.cpp

 //===-- GDBRemoteRegisterContext.cpp --------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
if (vg_reg_info) {
     uint64_t reg_value = ReadRegisterAsUnsigned(vg_reg_num, fail_value);
     if (reg_value != fail_value && reg_value <= 32)
       vg_reg_value = reg_value;
   }
 
   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)
       svg_reg_value = reg_value;
   }
 
   if (vg_reg_value)
     m_reg_info_sp->UpdateARM64SVERegistersInfos(*vg_reg_value);

lldb/test/API/commands/register/register/aarch64_za_register/za_dynamic_resize/TestZAThreadedDynamic.py

def za_test_impl(self, enable_za):
                 self.assertEqual(thread.GetStopReason(), lldb.eStopReasonBreakpoint)
 
             stopped_at_line_number = thread.GetFrameAtIndex(0).GetLineEntry().GetLine()
 
             if stopped_at_line_number == thX_break_line1:
                 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")
 
         for idx in range(1, process.GetNumThreads()):
             thread = process.GetThreadAtIndex(idx)
             self.assertEqual(thread.GetStopReason(), lldb.eStopReasonBreakpoint)