Hello, I also hit this bug on 32-bit mipsel, on the Malta platform in QEMU.
I used images from https://ftp.debian.org/debian/dists/Debian12.8/main/installer-mipsel/current/images/malta/netboot/ Command line: qemu-system-mipsel -cpu 24Kc -M malta -m 512 -kernel debian12.8/installer-mipsel/malta/vmlinuz-6.1.0-27-4kc-malta -initrd debian12.8/installer-mipsel/malta/initrd.gz -hda /media/1T-data/virtual_machines/debian_mips/hda.raw -append "root=/dev/sda1 nokaslr" -nographic I was able to work around it by doing the equivalent of: ~ # mkfs.ext4 /dev/sda1 ~ # mount /dev/sda1 /target ~ # cd /target /target # mkdir -p etc/initramfs-tools/conf.d /target # cd etc/initramfs-tools/conf.d/ /target/etc/initramfs-tools/conf.d # ls /target/etc/initramfs-tools/conf.d # echo 'COMPRESS=gzip' > compress /target/etc/initramfs-tools/conf.d # cd ~ # umount /target and having the installer not erase the partition during installation. I have run gdb and the offending instruction is in the ZSTD_RowFindBestMatch function and it is the "prefx" instruction. 0x555b02f8 <+1160>: addiu v0,v0,31 0x555b02fc <+1164>: andi v0,v0,0xf 0x555b0300 <+1168>: sll v0,v0,0x2 0x555b0304 <+1172>: addu v0,a3,v0 0x555b0308 <+1176>: lw v0,0(v0) 0x555b030c <+1180>: b 0x555b0368 <ZSTD_RowFindBestMatch_noDict_5_4+1272> 0x555b0310 <+1184>: move t7,v0 => 0x555b0314 <+1188>: prefx 0x6,t7(s5) 0x555b0318 <+1192>: subu v0,a1,v0 0x555b031c <+1196>: sw t7,0(a2) 0x555b0320 <+1200>: addiu t9,a0,-1 0x555b0324 <+1204>: and a1,a1,v0 0x555b0328 <+1208>: and a0,a0,t9 It appears that this instruction requires a floating-point coprocessor and is a CP1X instruction. It is used to prefetch locations from memory. Indeed, when using the 24Kf variant (qemu-system-mipsel -cpu 24Kf), zstd -9 works. The question is why the Linux kernel's math-emu module (which is compiled in and enabled) didn't catch and emulate it. root@mateusz-debian-mips:/sys/kernel/debug/mips# cat fpuemustats_clear root@mateusz-debian-mips:/sys/kernel/debug/mips# zstd -9 </etc/fstab >/dev/null Caught SIGILL signal, printing stack: Illegal instruction root@mateusz-debian-mips:/sys/kernel/debug/mips# grep -r . fpuemustats/* fpuemustats/branches:11 fpuemustats/cp1ops:44 fpuemustats/cp1xops:1 fpuemustats/ds_emul:1 fpuemustats/emulated:535 fpuemustats/errors:0 fpuemustats/ieee754_inexact:4 fpuemustats/ieee754_invalidop:0 fpuemustats/ieee754_overflow:0 fpuemustats/ieee754_underflow:0 fpuemustats/ieee754_zerodiv:0 [...] Greetings, Mateusz
