<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/107864>107864</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            lldb cannot read AArch64 FPU (Neon) registers from qemu when SVE is enabled
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            lldb
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          DavidSpickett
      </td>
    </tr>
</table>

<pre>
    ```
$ qemu-aarch64 -g 1234 /tmp/test.o

$ ./bin/lldb /tmp/test.o -o "log enable gdb-remote packets" -o "gdb-remote 1234"
<...>
(lldb) register read --all
<...>
general:
        x0 = 0x0000000000000000
<...>
  pauth_cmask_high = 0xffff000000000000
32 registers were unavailable.
(lldb) register read v0
error: Invalid register name 'v0'.
```
GDB 12.1 (only version I checked) does not list the registers in `info registers` but it will let you read it individually:
```
(gdb) info register v0
v0             {d = {f = {0x0, 0x0}, u = {0x0, 0x0}, s = {0x0, 0x0}}, s = {f = <...>
```
If I choose a CPU without SVE, the problem is fixed:
```
$ qemu-aarch64 -g 1234 -cpu cortex-a53 /tmp/test.o

(lldb) register read --all
general:
<...>
        v0 = {0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00}
<...>
       v31 = {0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00}
      fpsr = 0x00000000
      fpcr = 0x00000000

(lldb) register read v0
      v0 = {0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00}
```
This is because QEMU does not send info about the floating point registers if SVE is enabled. On the grounds that we should know that we can just extract them from the bottom 128 bits of the SVE registers.

I might have argued that that's exactly what the XML is supposed to inform us about, but ok, that ship has sailed.

https://gitlab.com/qemu-project/qemu/-/blob/master/target/arm/gdbstub.c?ref_type=heads#L512

```
 /*
         * The lower part of each SVE register aliases to the FPU
         * registers so we don't need to include both.
 */
```
QEMU sends us XML like this:
```
<?xml version="1.0"?><!DOCTYPE target SYSTEM "gdb-target.dtd">
<target>
<architecture>aarch64</architecture>
<xi:include href="aarch64-core.xml"/>
<xi:include href="sve-registers.xml"/>
<xi:include href="aarch64-pauth.xml"/>
<xi:include href="system-registers.xml"/>
</target>
```
The SVE contents are:
```
<?xml version="1.0"?>
<!DOCTYPE feature SYSTEM "gdb-target.dtd">
<feature name="org.gnu.gdb.aarch64.sve">
 <...>
  <reg name="z0" bitsize="2048" regnum="34" type="svev"/>
  <...>
  <reg name="fpsr" bitsize="32" regnum="66" type="int" group="float"/>
  <reg name="fpcr" bitsize="32" regnum="67" type="int" group="float"/>
  <reg name="p0" bitsize="256" regnum="68" type="svep"/>
  <...>
 <reg name="ffr" bitsize="256" regnum="84" type="svep" group="vector"/>
  <reg name="vg" bitsize="64" regnum="85" type="int"/>
</feature>
```
It does not include specific entries for `v0` and the other FPU registers.

Clearly lldb knows there are 32 other registers (v0-31) that we expect to get, so some fallback code is being executed. It just doesn't know where to get the values from. We'll have to teach that to use the SVE values.
</pre>
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