Re: [PULL 81/94] target/sparc: Move FDMULQ to decodetree

[Mark Cave-Ayland]

On 07/11/2023 04:49, Richard Henderson wrote:

> On 11/6/23 14:02, Mark Cave-Ayland wrote:
> > I was working through my SPARC boot tests for your latest target/sparc series when 
> > I spotted a segfault on my FreeBSD SPARC64 image. A git bisect indicated that this 
> > was the patch that originally introduced the error, something I must have missed 
> > when testing the original decodetree conversion series.
> >
> > The reproducer is:
> >
> > ./qemu-system-sparc64 -m 256 -cdrom FreeBSD-10.3-RELEASE-sparc64-bootonly.iso \
> >      -boot d -nographic
> >
> > and the error is a segfault in devd:
> >
> > ...
> > ...
> > Trying to mount root from cd9660:/dev/iso9660/10_3_RELEASE_SPARC64_BO [ro]...
> > Entropy harvesting: interrupts ethernet point_to_point swi.
> > Starting file system checks:
> > Mounting local file systems:.
> > Writing entropy file:.
> > /etc/rc: WARNING: $hostname is not set -- see rc.conf(5).
> > Starting Network: lo0 hme0.
> > lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384
> >          options=600003<RXCSUM,TXCSUM,RXCSUM_IPV6,TXCSUM_IPV6>
> >          inet6 ::1 prefixlen 128
> >          inet6 fe80::1%lo0 prefixlen 64 scopeid 0x2
> >          inet 127.0.0.1 netmask 0xff000000
> >          nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
> > hme0: flags=8802<BROADCAST,SIMPLEX,MULTICAST> metric 0 mtu 1500
> >          options=8000b<RXCSUM,TXCSUM,VLAN_MTU,LINKSTATE>
> >          ether 52:54:00:12:34:56
> >          nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
> >          media: Ethernet autoselect
> > Starting devd.
> > pid 246 (ps), uid 0: exited on signal 11
> > Segmentation fault
> > ^^^^^^^^^^^^^^^^^^
>
>
> I certainly can't imagine that FdMULq is really at fault, because it's not 
> implemented on real hardware (and thus I really doubt FreeBSD attempted to use it), 
> and CPU_FEATURE_FLOAT128 is not enabled by your command-line.
>
> The only thing that I can imagine is that this is some sort of timing related issue 
> and bisect behaved randomly.

Hmmm you're right, it seems that there is a semi-random aspect to the issue which is 
why the bisection didn't give a good result.

In order to mitigate this, I repeated the bisection again but this time booting the 
FreeBSD image 5 times in a row, only marking the commit as good if all 5 boot tests 
passed [1] without displaying a segfault message [2]. That bisection led me to this 
commit:


86b82fe021f46ed4501b16132f7e3fccd0a1ad5d is the first bad commit
commit 86b82fe021f46ed4501b16132f7e3fccd0a1ad5d
Author: Richard Henderson <richard.henderson@linaro.org>
Date:   Wed Oct 4 17:51:37 2023 -0700

     target/sparc: Move JMPL, RETT, RETURN to decodetree

     Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
     Acked-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
     Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

  target/sparc/insns.decode |   7 +++
  target/sparc/translate.c  | 126 +++++++++++++++++++++++++++++-----------------
  2 files changed, 88 insertions(+), 45 deletions(-)


I tried repeating the boot 5 times in a row test on both this commit and the commit 
before it, and that appeared to confirm the bisection result with the failures only 
appearing with commit 86b82fe021.

> All that said, I can't replicate this with master.
> Can you, now?

Yes, the issue is still present in master for me. My host is Debian bookworm (stable, 
x86_64) configured as './configure' '--target-list=sparc64-softmmu' '--enable-slirp' 
'--enable-debug' and the command line is:

./qemu-system-sparc64 -m 256 -cdrom FreeBSD-10.3-RELEASE-sparc64-bootonly.iso \
     -boot d -nographic


ATB,

Mark.

[1] Sometimes the image hangs just after "Trying to mount root .." but that appears 
to be a timing issue and not directly related to this series. In this cases where 
this happened, I simply quit QEMU and rebooted the image again.

[2] Normally the segfault message comes from devd, but during the bisection I did 
occasionally see it coming from other processes.