Re: [Xen-devel] Bug in x86 instruction emulator?

["William Z." <wogiz@xxxxxxxxxxxxxxx>]

On 2016-05-18 11:12, Jan Beulich wrote:
> > > > On 06.04.16 at 01:38, <wogiz@xxxxxxxxxxxxxxx> wrote:
> > I'm running Xen 4.6.1 with Alpine Linux 3.3.3 in dom0. In a HVM domU
> > with vga="qxl", Xorg will segfault instantly if tried started. 
> > Multiple
> > Linux distros have been tested and Xorg segfaults in all.
> >
> > Attached are a full backtrace from domU generated by Xorg, and a
> > assembler dump of function 'sse2_blt'.
> Just FYI: Looks like I can repro this finally, and it also looks like 
> at
> least for me it isn't an SSE2 instruction that the issue is with.
> Instead I'm getting an #UD in the middle of an instruction a few
> lines down from the last SSE2 one, which suggests we're having
> an issue with sizing instructions (however odd that may seem).
> Now that I can repro it, at least I have something to actually
> debug ...
>
> Jan
I have patched Xen 4.6.1 with commit 
2bb230972c5ddb1ca823f47750b5d46a9d302d0e
(x86emul: suppress writeback upon unsuccessful MMX/SSE/AVX insn 
emulation) and
tested with different Linux distros. I can say with confidence that the 
patch
has solved my initial problem as Xorg no longer segfaults when started. 
Thanks
to everyone that has helped with this.

However, while testing I have found a new problem. This may not be 
related to
my initial problem or even Xen, but I will try to describe it here as 
I'm hoping
someone can point me in the right direction.

Various actions will now raise the CPU usage of Xorg to 100% and freeze 
the
entire X Window System for some time. E.g.:

Starting xterm in a window manager or directly from .xinitrc and 
executing
dmesg. This will print a few lines per second while the Xorg CPU usage 
is 100%
and the X Window System is frozen for about 60 seconds until all dmesg 
output
has been printed.

I have run 'perf record -g -a sleep 60' while connected via SSH and then
executed dmesg in xterm. I have attached a few lines of 'perf report -g' 
with
the first one expanded.

I have also run 'strace -p $(pidof Xorg)' while dmesg was running in 
xterm. The
lines I have attached will repeat until all dmesg output has been 
printed. File
descriptor 8 is pointing on '/dev/dri/card0'.

Any ideas on what could cause this?

William Z.

Attachment: perf_report.txt
Description: Text document

Attachment: strace.txt
Description: Text document

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