[systemd-devel] rdrand generated with march=winchip-c6 in systemd-241

tedheadster tedheadster at gmail.com
Sat May 11 17:19:03 UTC 2019

On Sat, May 11, 2019 at 12:30 PM Florian Weimer <fw at deneb.enyo.de> wrote:
> Can you capture register contents at the point of the crash?
> Does this reproduce in a chroot?  Maybe you can trace the whole thing
> with a debugger.  Does the crash reproduce if you single-step through
> the whole function?

  I figured out the problem, I just haven't written code to fix it.
The documentation I can find is silent about what is returned in %ecx
and %ebx when calling cpuid function 0x00000001 on IDT Winchip-C6 and

I think %ecx  should properly contain 0x00000000, but it instead puts
the 'auls' characters from cpuid function 0x00000000 (vendor string
'CentaurHauls') in %ecx:

%ebx = 0x746e6543 = "Cent"
%edx = 0x48727561 = "aurH"
%ecx = 0x736c7561 = "auls"

This sets bit 30 (0x736c7561) 'on', the 'supports rdrand' bit.

So we have to code around the vendor and chip model in this case.
Jeffrey Walton gave some coding examples I might consider

- Matthew

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