This bug was fixed in the package qemu - 1:5.0-5ubuntu4
---------------
qemu (1:5.0-5ubuntu4) groovy; urgency=medium
* xen: provide compat links to what libxen-dev reports where to find
the binaries (LP: #1890005)
* d/p/ubuntu/lp-1883984-target-s390x-Fix-SQXBR.patch: avoid crash on
SQXBR (LP: #1883984)
* d/p/lp-1890154-*: fix -no-reboot on s390x secure boot (LP: #1890154)
-- Christian Ehrhardt <christian.ehrha...@canonical.com> Mon, 03 Aug
2020 07:15:28 +0200
** Changed in: qemu (Ubuntu)
Status: In Progress => Fix Released
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https://bugs.launchpad.net/bugs/1883984
Title:
QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system-
s390x
Status in QEMU:
Fix Committed
Status in qemu package in Ubuntu:
Fix Released
Status in qemu source package in Focal:
Triaged
Bug description:
In porting software to guest Ubuntu 18.04 and 20.04 VMs for S/390x, I
discovered
that some of my own numerical programs, and also a GNU configure script for at
least one package with CC=clang, would cause an instant crash of the VM,
sometimes
also destroying recently opened files, and producing long strings of NUL
characters
in /var/log/syslog in the S/390 guest O/S.
Further detective work narrowed the cause of the crash down to a single IBM
S/390
instruction: sqxbr (128-bit IEEE 754 square root). Here is a one-line program
that when compiled and run on a VM hosted on QEMUcc emulator version 4.2.0
(Debian 1:4.2-3ubuntu6.1) [hosted on Ubuntu 20.04 on a Dell Precision 7920
workstation with an Intel Xeon Platinum 8253 CPU], and also on QEMU emulator
version 5.0.0, reproducibly produces a VM crash under qemu-system-s390x.
% cat bug-sqrtl-one-line.c
int main(void) { volatile long double x, r; x = 4.0L; __asm__
__volatile__("sqxbr %0, %1" : "=f" (r) : "f" (x)); return (0);}
% cc bug-sqrtl-one-line.c && ./a.out
Segmentation fault (core dumped)
The problem code may be the function float128_sqrt() defined in
qemu-5.0.0/fpu/softfloat.c
starting at line 7619. I have NOT attempted to run the qemu-system-s390x
executable
under a debugger. However, I observe that S/390 is the only CPU family that
I know of,
except possibly for a Fujitsu SPARC-64, that has a 128-bit square root in
hardware.
Thus, this instruction bug may not have been seen before.
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