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 to learn about the extension and here 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. Making each one vector length / 8 sized.

  • 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 to learn about the extension and here 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.


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 dissappear). 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 instrucion 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.