# Using LLDB On AArch64 Linux

This page explains the details of debugging certain AArch64 extensions using
LLDB. If something is not mentioned here, it likely works as you would expect.

This is not a replacement for ptrace and Linux Kernel documentation. This covers
how LLDB has chosen to use those things and how that effects your experience as
a user.

## Scalable Vector Extension (SVE)

See [here](https://developer.arm.com/Architectures/Scalable%20Vector%20Extensions)
to learn about the extension and [here](https://kernel.org/doc/html/latest/arch/arm64/sve.html)
for the Linux Kernel's handling of it.

In LLDB you will be able to see the following new registers:

* `z0-z31` vector registers, each one has size equal to the vector length.
* `p0-p15` predicate registers, each one containing 1 bit per byte in the vector
  length. So each one is `vector length in bits / 8` bits.
* `ffr` the first fault register, same size as a predicate register.
* `vg`, the vector length in "granules". Each granule is 8 bytes.

```
       Scalable Vector Extension Registers:
             vg = 0x0000000000000002
             z0 = {0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 <...> }
           <...>
             p0 = {0xff 0xff}
           <...>
            ffr = {0xff 0xff}
```

The example above has a vector length of 16 bytes. Within LLDB you will always
see "vg" as in the `vg` register, which is 2 in this case (8*2 = 16).
Elsewhere in kernel code or applications, you may see "vq" which is the vector
length in quadwords (16 bytes). Where you see "vl", it is in bytes.

While you can count the size of a P or Z register, it is intended that `vg` be
used to find the current vector length.

### Changing the Vector Length

The `vg` register can be written during a debug session. Writing the current
vector length changes nothing. If you increase the vector length, the registers
will likely be reset to 0. If you decrease it, LLDB will truncate the Z
registers but everything else will be reset to 0.

You should not assume that SVE state after changing the vector length is in any
way the same as it was previously. Whether that is done from within the
debuggee, or by LLDB. If you need to change the vector length, do so before a
function's first use of SVE.

### Z Register Presentation

LLDB makes no attempt to predict how SVE Z registers will be used. Since LLDB
does not know what sort of elements future instructions will interpret the
register as. It therefore does not change the visualisation of the register
and always defaults to showing a vector of byte sized elements.

If you know what format you are going to use, give a format option:
```
  (lldb) register read z0 -f uint32_t[]
      z0 = {0x01010101 0x01010101 0x01010101 0x01010101}
```

### FPSIMD and SVE Modes

Prior to the debugee's first use of SVE, it is in what the Linux Kernel terms
SIMD mode. Only the FPU is being used. In this state LLDB will still show the
SVE registers however the values are simply the FPU values zero extended up to
the vector length.

On first access to SVE, the process goes into SVE mode. Now the Z values are
in the real Z registers.

You can also trigger this with LLDB by writing to an SVE register. Note that
there is no way to undo this change from within LLDB. However, the debugee
itself could do something to end up back in SIMD mode.

### Expression evaluation

If you evaluate an expression, all SVE state is saved prior to, and restored
after the expression has been evaluated. Including the register values and
vector length.

## Scalable Matrix Extension (SME)

See [here](https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/scalable-matrix-extension-armv9-a-architecture)
to learn about the extension and [here](https://kernel.org/doc/html/latest/arch/arm64/sme.html)
for the Linux Kernel's handling of it.

SME adds a "Streaming Mode" to SVE, and this mode has its own vector length
known as the "Streaming Vector Length".

In LLDB you will see the following new registers:

* `tpidr2`, an extra per thread pointer reserved for use by the SME ABI.
  This is not scalable, just pointer sized aka 64 bit.
* `z0-z31` streaming SVE registers. These have the same names as the
  non-streaming registers and therefore you will only see the active set in
  LLDB. You cannot read or write the inactive mode's registers. Their size
  is the same as the streaming vector length.
* `za` the Array Storage register. The "Matrix" part of "Scalable Matrix
  Extension". This is a square made up of rows of length equal to the streaming
  vector length (svl). Meaning that the total size is svl * svl.
* `svcr` the Streaming Vector Control Register. This is actually a pseduo
  register but it matches the content of the architecturaly defined `SVCR`.
  This is the register you should use to check whether streaming mode and/or
  `za` is active. This register is read only.
* `svg` the streaming vector length in granules. This value is not connected
  to the vector length of non-streaming mode and may change independently. This
  register is read only.

```{note}
  While in non-streaming mode, the `vg` register shows the non-streaming
  vector length, and the `svg` register shows the streaming vector length.
  When in streaming mode, both `vg` and `svg` show the streaming mode vector
  length. Therefore it is not possible at this time to read the non-streaming
  vector length within LLDB, while in streaming mode. This is a limitation of
  the LLDB implementation not the architecture, which stores both lengths
  independently.
```

In the example below, the streaming vector length is 16 bytes and we are in
streaming mode. Note that bits 0 and 1 of `svcr` are set, indicating that we
are in streaming mode and ZA is active. `vg` and `svg` report the same value
as `vg` is showing the streaming mode vector length:
```
  Scalable Vector Extension Registers:
        vg = 0x0000000000000002
        z0 = {0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 <...> }
        <...>
        p0 = {0xff 0xff}
        <...>
       ffr = {0xff 0xff}

  <...>

  Thread Local Storage Registers:
       tpidr = 0x0000fffff7ff4320
      tpidr2 = 0x1122334455667788

  Scalable Matrix Extension Registers:
         svg = 0x0000000000000002
        svcr = 0x0000000000000003
          za = {0x00 <...> 0x00}
```

### Changing the Streaming Vector Length

To reduce complexity for LLDB, `svg` is read only. This means that you can
only change the streaming vector length using LLDB when the debugee is in
streaming mode.

As for non-streaming SVE, doing so will essentially make the content of the SVE
registers undefined. It will also disable ZA, which follows what the Linux
Kernel does.

### Visibility of an Inactive ZA Register

LLDB does not handle registers that can come and go at runtime (SVE changes
size but it does not disappear). Therefore when `za` is not enabled, LLDB
will return a block of 0s instead. This block will match the expected size of
`za`:
```
  (lldb) register read za svg svcr
      za = {0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 <...> }
     svg = 0x0000000000000002
    svcr = 0x0000000000000001
```

Note that `svcr` bit 2 is not set, meaning `za` is inactive.

If you were to write to `za` from LLDB, `za` will be made active. There is
no way from within LLDB to reverse this change. As for changing the vector
length, the debugee could still do something that would disable `za` again.

If you want to know whether `za` is active or not, refer to bit 2 of the
`svcr` register, otherwise known as `SVCR.ZA`.

### ZA Register Presentation

As for SVE, LLDB does not know how the debugee will use `za`, and therefore
does not know how it would be best to display it. At any time any given
instruction could interpret its contents as many kinds and sizes of data.

So LLDB will default to showing `za` as one large vector of individual bytes.
You can override this with a format option (see the SVE example above).

### Expression Evaluation

The mode (streaming or non-streaming), streaming vector length and ZA state will
be restored after expression evaluation. On top of all the things saved for SVE
in general.

## Scalable Matrix Extension (SME2)

The Scalable Matrix Extension 2 is documented in the same architecture
specification as SME, and covered by the same kernel documentation page as SME.

SME2 adds 1 new register, `zt0`. This register is a fixed size 512 bit
register that is used by new instructions added in SME2. It is shown in LLDB in
the existing SME register set.

`zt0` can be active or inactive, as `za` can. The same `SVCR.ZA` bit
controls this. An inactive `zt0` is shown as 0s, like `za` is. Though in
`zt0`'s case, LLDB does not need to fake the value. Ptrace already returns a
block of 0s for an inactive `zt0`.

Like `za`, writing to an inactive `zt0` will enable it and `za`. This can
be done from within LLDB. If the write is instead to `za`, `zt0` becomes
active but with a value of all 0s.

Since `svcr` is read only, there is no way at this time to deactivate the
registers from within LLDB (though of course a running process can still do
this).

To check whether `zt0` is active, refer to `SVCR.ZA` and not to the value of
`zt0`.

### ZT0 Register Presentation

As for `za`, the meaning of `zt0` depends on the instructions used with it,
so LLDB does not attempt to guess this and defaults to showing it as a vector of
bytes.

### Expression Evaluation

`zt0`'s value and whether it is active or not will be saved prior to
expression evaluation and restored afterwards.

## SME Only Systems

AArch64 systems may have both SVE and SME, or they can have only SME. If they
only have SME, the system has the usual SVE state, but that state and SVE
instructions may only be used while in streaming mode.

The LLDB experience is very similar to SVE+SME systems, with a few changes.

### Registers

When in streaming mode, registers act as they would on an SVE+SME system.
Outside of streaming mode LLDB will show the `Z` registers as zero extended copies
of the `V` registers.

Writes to `Z` registers are allowed, but these are converted to `V` register writes
and so only the bottom 128-bits will be applied. The size of the value
written to a `Z` register must match the current streaming vector length, even if
the process is in non-streaming mode.

For an SME only system, `lldb-server` describes the system as having `Z`
registers with `V` registers as subsets of those registers. We do not change the
representation each time the mode changes.

A consequence of this is that if the user writes to a `V` register while in
non-streaming mode, it will be sent to `lldb-server` as a `Z` register write
of a zero extended value. `lldb-server` will convert that back into a `V`
register write with the value truncated to 128-bits.

In streaming mode, it will be zero extended, sent as a `Z` write and written
to the real streaming SVE `Z` register without truncation.

`P` registers will be shown as 0s in non-streaming mode. They cannot be
written to in non-streaming mode.

The `ffr` register will also be shown as 0s in non-streaming mode. In streaming
mode, use of `ffr` is forbidden so it will also show as 0s. It cannot be written
to in either state.

(in the former state, LLDB generates the fake value, in the latter state, the
kernel tells `lldb-server` that the value is 0, the result is the same)

The `ZA` and `ZT0` registers act as they would for an SVE+SME system.

Since there is no non-streaming SVE, there is no non-streaming vector length.
Therefore even in non-streaming mode, the value shown in `vg` will be the
streaming vector length, and it will be equal to the value of `svg`.

### Expression Evaluation

Some instructions are illegal to use in streaming mode, unless `FEAT_SMEFA64`
is present.

LLDB **will not** make any attempt to make expressions compatible with the
current mode.

If part of an expression is not compatible, it will
result in a `SIGILL` that will be cleaned up as any other signal would be. All
register, `ZA` and mode state will be restored as normal after expression
evaluation.

Note that to restore to a non-streaming state from a streaming state, LLDB uses
a special part of the Linux Kernel's
[SVE ABI](https://docs.kernel.org/arch/arm64/sve.html). FPSIMD format data is
written to the non-existent non-streaming SVE register set, with the vector
length set to 0 to cause the process to exit streaming mode and apply those
FPSIMD values to the `V` registers.

This feature was added in version 6.19 of the Linux kernel. On older versions,
LLDB will attempt the restore and it will fail. This will result in the mode
and some register state not being restored.

This feature is only used for this purpose. Otherwise, in non-streaming mode FP
registers are accessed using the FP register set, and in streaming mode using
the streaming SVE register set.

## Guarded Control Stack Extension (GCS)

GCS support includes the following new registers:

* `gcs_features_enabled`
* `gcs_features_locked`
* `gcspr_el0`

These map to the registers ptrace provides. The first two have a `gcs_`
prefix added as their names are too generic without it.

When the GCS is enabled the kernel allocates a memory region for it. This region
has a special attribute that LLDB will detect and presents like this:
```
  (lldb) memory region --all
  <...>
  [0x0000fffff7a00000-0x0000fffff7e00000) rw-
  shadow stack: yes
  [0x0000fffff7e00000-0x0000fffff7e10000) ---
```

`shadow stack` is a generic term used in the kernel for secure stack
extensions like GCS.

### Expression Evaluation

To execute an expression when GCS is enabled, LLDB must push the return
address of the expression wrapper (usually the entry point of the program)
to the Guarded Control Stack. It does this by decrementing `gcspr_el0` and
writing to the location now pointed to by `gcspr_el0` (instead of using the
GCS push instructions).

After an expression finishes, LLDB will restore the contents of all 3
GCS registers, apart from the enable bit of `gcs_features_enabled`. This is
because there are limits on how often and from where you can set this
bit.

GCS cannot be enabled from ptrace and it is expected that a process which
has enabled and then disabled GCS, will not enable it again. The simplest
choice was to not restore the enable bit at all. It is up to the user or
program to manage that bit.

The return address that LLDB pushed onto the Guarded Control Stack will be left
in place. As will any values that were pushed to the stack by functions run
during the expression.

When the process resumes, `gcspr_el0` will be pointing to the original entry
on the guarded control stack. So the other values will have no effect and
likely be overwritten by future function calls.

LLDB does not track and restore changes to general memory during expressions,
so not restoring the GCS contents fits with the current behaviour.

Note that if GCS is disabled and an expression enables it, LLDB will not
be able to setup the return address and it is up to that expression to do that
if it wants to return to LLDB correctly.

If it does not do this, the expression will fail and although most process
state will be restored, GCS will be left enabled. Which means that the program
is very unlikely to be able to progress.
