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id = 1955
title = "powerpc instruction 'mffsl' not emulated on POWER8"
state = "closed"
created_at = "2023-10-22T16:59:43.540Z"
closed_at = "2023-10-25T05:44:08.050Z"
labels = ["target: ppc"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/1955"
host-os = "Ubuntu 22.04"
host-arch = "x86_64"
qemu-version = "8.0.2"
guest-os = "Debian 12"
guest-arch = "ppc, ppc64"
description = """Since 2019, the function feenableexcept() in GNU libc makes use of the "mffsl" instruction.
See https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/powerpc/fpu/feenablxcpt.c;h=b111ceaa4e2e1864fcbe043ccda34e03e9f14062;hb=HEAD#l28
and https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/powerpc/fpu/fenv_libc.h;h=a2a12d914b47e99746003482b349a0675cc5ad34;hb=HEAD#l57
In the emulated Debian system, executables that make use of this instruction crash with SIGILL.
Likewise, under gdb (in the emulated system), there is a SIGILL at the 'mffsl' instruction.
From the comments in the above glibc source, added by Paul A. Clarke <pc@us.ibm.com>:
"Nicely, it turns out that the 'mffsl' instruction will decode to
'mffs' on architectures older than "power9" because the additional
bits set for 'mffsl' are "don't care" for 'mffs'. 'mffs' is a superset
of 'mffsl'."
This is indeed what I observe by compiling and running the attached program foo.c on a hardware machine with a POWER8 CPU: That program does not crash with a SIGILL."""
reproduce = """1. Either run the attached 'test-fenv-except-tracking-5.ppc' (32-bit) program under qemu-system-ppc.
2. Or run the the attached 'test-fenv-except-tracking-5.ppc64' (64-bit) program under qemu-system-ppc64 with -cpu POWER8.
3. Or compile and run the attached foo.c and run it under QEMU."""
additional = """[test-fenv-except-tracking-5.ppc.xz](/uploads/8222ebac115e8a865d5e520b25d423ff/test-fenv-except-tracking-5.ppc.xz)
[test-fenv-except-tracking-5.ppc64.xz](/uploads/d0522723541a46e11ab55b8f45dfb574/test-fenv-except-tracking-5.ppc64.xz)
[foo.c](/uploads/35d8b3b1e5b39ecb6a2a899132858ded/foo.c)"""
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