Re: xenstored crashes with SIGSEGV

[Philipp Hahn <hahn@univention.de>]

Hello,

On 17.12.2014 10:14, Frediano Ziglio wrote:
> 2014-12-16 16:44 GMT+00:00 Frediano Ziglio <freddy77@gmail.com>:
>> 2014-12-16 16:23 GMT+00:00 Ian Campbell <Ian.Campbell@citrix.com>:
...
>> First we (I'll try when I reach home) can check if memset in glibc (or
>> the version called from talloc_zero) can use SSE. A possible dmesg
>> output and /proc/cpuinfo content could help too but I think SSE are
>> now quite common.
> 
> I have access to some core dumps. glibc memset is using SSE,
> specifically xmm0 register.
> 
> Unfortunately is seems that core dumps contains only standard
> registers, so all register appears zeroed. If you try with a newer gdb
> version is shows that registers are not available.

I had another look myself and I'm confused now:

Using "info float" or "info vector" with gdb-7.0.1 shows the FP and MMX
registers to be all zero.
A newer gdb-7.2 shows the registers as "unavailable".

"eu-readelf --notes core" doesn't show a NT_FPREGSET note, so to me it
looks like at least the FP-registers were not dumped.
But is that also used for the MMX registers? If my memory is right, the
FP and MMX registers are "shared" in the CPU, but that might be old
knowledge.

I wrote a small SSE using program, which dumps core. If I run that
locally and do a "readelf --notes core", I get:
  CORE          0x00000200      NT_FPREGSET (floating point registers)

If I do the same in dom0, I don't get that note and gdb doesn't show the
register content.
SSE seems to be available in the dom0, as the program would crash with
SIGILL otherwise:
# grep ^flags /proc/cpuinfo
flags           : fpu de tsc msr pae mce cx8 apic sep mca cmov pat
clflush acpi mmx fxsr sse sse2 ss ht syscall nx lm constant_tsc rep_good
nopl nonstop_tsc pni est ssse3 cx16 sse4_1 sse4_2 popcnt hypervisor
lahf_lm ida dtherm

Look like that got fixed with a newer 3.10.61 kernel, so I'll urge our
admins to update to a later kernel (again), so we'll get more useful
core dumps for future crashes.

I'm still investigating the core files of the other programs, but it
takes some time. I don't know if I will be able to finish that in time,
as the Christmas holiday season starts tomorrow and I will be
unavailable for nearly two weeks,

So happy Christmas to everybody and thanks again for your help.

Philipp