Illegal instruction on x86 Qemu guest

[svillemot]

The OpenBLAS 0.2.8 Debian package for i386 (x86) fails to build. A test crashes with illegal instruction. The full build log is there:

https://buildd.debian.org/status/fetch.php?pkg=openblas&arch=i386&ver=0.2.8-1&stamp=1375567538

The machine on which the build was performed is a Qemu guest. Here is the contents of its /proc/cpuinfo (repeated 8 times):

processor       : 7
vendor_id       : AuthenticAMD
cpu family      : 6
model           : 2
model name      : QEMU Virtual CPU version 1.1.2
stepping        : 3
microcode       : 0x1000065
cpu MHz         : 2100.000
cache size      : 512 KB
fpu             : yes
fpu_exception   : yes
cpuid level     : 4
wp              : yes
flags           : fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx lm nopl pni cx16 popcnt hypervisor lahf_lm svm abm sse4a
bogomips        : 4200.00
TLB size        : 1024 4K pages
clflush size    : 64
cache_alignment : 64
address sizes   : 40 bits physical, 48 bits virtual
power management:

I don't have access to the machine. However, if needed, I could try to replicate the problem by creating a similar Qemu guest.

[xianyi]

Hi @sebastien-villemot ,

Thank you for the feedback.

I didn't meet this error before. Please reproduce this problem on your Qemu guest.

I need the backtrace and the exe file. Then, I can objdump the exe to investigate the instruction.

Xianyi

[cartazio]

I don't see xsave on the list of cpu extensions, isn't xsave needed for simd registers to work well with function calls?

[xianyi]

I think xsave is for AVX instructions. Qemu looks like a SSE4a CPU cores.

[svillemot]

I have found the cause of this problem. The Qemu guest mentioned above is recognized as an Athlon by the dynamic engine of OpenBLAS (because of its CPU vendor and model). However, the virtual CPU does not have the 3dnow! extension. Hence OpenBLAS crashes when trying to use that set of instructions.

I have therefore patched the Debian package for OpenBLAS in such a way that it effectively tests the presence of 3dnow! extension before deciding it is an Athlon CPU. Otherwise, it fallbacks to Katmai.

The patch is available there: http://anonscm.debian.org/gitweb/?p=debian-science/packages/openblas.git;a=blob;f=debian/patches/qemu-athlon.patch;h=51174b97098e9b2191bde9210e9ca01f4911b275;hb=HEAD

Tell me if you are willing to incorporate it into OpenBLAS, in which case I'll do a pull request.

[xianyi]

Hi @sebastien-villemot ，

Good work!~
Thank you for the investigation. Please do a pull request.

Xianyi