Re: gdb support for SME-without-SVE ?

[Thiago Jung Bauermann via Gdb <gdb@sourceware.org>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On Thu, 29 Jan 2026 at 09:10, Peter Maydell <peter.maydell@linaro.org> wrote:
>>
>> On Thu, 29 Jan 2026 at 04:12, Thiago Jung Bauermann
>> <thiago.bauermann@linaro.org> wrote:
>> >
>> > Peter Maydell <peter.maydell@linaro.org> writes:
>> >
>> > > On Wed, 28 Jan 2026 at 04:24, Thiago Jung Bauermann
>> > > <thiago.bauermann@linaro.org> wrote:
>> > >> I'm currently implementing SME-only (i.e. without SVE) support in GDB
>> > >> for Linux userspace debugging. It's mostly done, I just need to test a
>> > >> few things, polish the code and prepare the patches. I'm hoping to send
>> > >> the patches next week.
>> > >
>> > > Let me know if a QEMU setup emulating "SME only" would be helpful
>> > > for you with that.
>> >
>> > It would indeed! If there's a branch I can build, it would make my
>> > testing quicker, and the regression testing more complete.
>>
>> I'll get one sorted out later today (the patches I have right now
>> have a few issues I need to fix).
>
> This branch:
>
> https://gitlab.com/pm215/qemu/-/commits/sme-no-sve

Thank you very much!

> should let you create an SME-only CPU with "-cpu max,sve=off".
> I have not given it much more than a very cursory testing, so
> let me know if you run into any problems with it.

Unfortunately I hit a kernel oops reliably when trying to run a GDB testcase:

[  156.387143] Internal error: Oops - Undefined instruction: 0000000002000000 [#1]  SMP
[  156.389966] Modules linked in: sm3_ce tpm_tis tpm_tis_core sha256 cfg80211 rfkill fuse drm backlight dm_mod ipv6 btrfs blake2b libblake2b xor xor_neon raid6_pq zstd_compress
[  156.391005] CPU: 0 UID: 1000 PID: 677 Comm: aarch64-sme-san Not tainted 6.19.0-rc5 #4 PREEMPT
[  156.391244] Hardware name: QEMU QEMU Virtual Machine, BIOS edk2-stable202408-prebuilt.qemu.org 08/13/2024
[  156.391475] pstate: 214020c5 (nzCv daIF +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
[  156.391657] pc : sve_get_vl+0x4/0xc
[  156.392149] lr : fpsimd_save_user_state+0x100/0x1b8
[  156.392270] sp : ffff800080cbbb80
[  156.392329] x29: ffff800080cbbb80 x28: 0000000000000001 x27: fff00000c44b05e0
[  156.392501] x26: ffff800080cbbdf0 x25: 0000000000000000 x24: 0000000000000000
[  156.392627] x23: 0000000000000000 x22: fff05a09d4047000 x21: 0000000000000020
[  156.392745] x20: fff00000ff5380f0 x19: ffffa5f72b4f10f0 x18: 0000000000000000
[  156.392870] x17: 0000000000000000 x16: 0000000000000000 x15: 02ed5d640eed9128
[  156.392994] x14: 0000b00e8e55cf08 x13: 000000000000033b x12: 000000000000033b
[  156.393109] x11: 0000000000000000 x10: 0000000000000002 x9 : 0000000000000800
[  156.393246] x8 : fff00000c36aac80 x7 : fff00000c36aac00 x6 : 00000007fb30b524
[  156.393372] x5 : 0000000000000000 x4 : 0000000000000001 x3 : ffff800080cbbc60
[  156.393491] x2 : 0000000000000000 x1 : fff00000c44b0000 x0 : 0000000000000001
[  156.393681] Call trace:
[  156.393853]  sve_get_vl+0x4/0xc (P)
[  156.394027]  fpsimd_thread_switch+0x44/0x188
[  156.394125]  __switch_to+0x20/0x1a8
[  156.394203]  __schedule+0x368/0xcdc
[  156.394270]  schedule+0x34/0x110
[  156.394328]  ptrace_stop.part.0+0x12c/0x24c
[  156.394409]  get_signal+0x384/0x82c
[  156.394471]  arch_do_signal_or_restart+0x118/0x3c4
[  156.394555]  exit_to_user_mode_loop+0x104/0x16c
[  156.394640]  el0_undef+0x88/0x8c
[  156.394704]  el0t_64_sync_handler+0x70/0xe4
[  156.394779]  el0t_64_sync+0x198/0x19c
[  156.394973] Code: b9400424 d51b4404 d65f03c0 d503245f (04bf5020)
[  156.395301] ---[ end trace 0000000000000000 ]---

The same kernel and userspace binaries work fine in Arm FVP.

In case you want to try it, this is my GDB branch:

https://gitlab.com/bauermann/binutils-gdb/-/commits/sme-only

And you can run the testcase with:

$ cd /path/to/gdb/build/dir; make check-gdb TESTS=gdb.arch/aarch64-sme-sanity.exp

I can try to create a minimal reproducer if it will help.

My QEMU command line is:

$QEMU_PREFIX/bin/qemu-system-aarch64 \
    -M virt \
    -cpu max,sve=off \
    -m 4g \
    -drive if=none,file=$HOME/VMs/ubuntu-aarch64.img,id=hd1,format=raw,cache=writeback,discard=on \
    -device virtio-blk-device,drive=hd1 \
    -netdev user,id=mynet0,hostfwd=tcp::8022-:22 -device virtio-net-pci,netdev=mynet0 \
    -bios $QEMU_PREFIX/share/qemu/edk2-aarch64-code.fd \
    -nographic

-- 
Thiago