Re: xsave=0 workaround needed on 3.2 kernels with Xen 4.1 or Xen-unstable.

[Jeff Law <law@redhat.com>]

On 05/09/2012 11:38 AM, Konrad Rzeszutek Wilk wrote:
>>
>> And finally one always has to keep in mind that there is this nice
>> glibc bug in that in some versions it is failing to look at CPUID.OSXSAVE
>> when trying to determine whether AVX or FMA is available.
>
> It has this:
> 146
> 147   if (__cpu_features.cpuid[COMMON_CPUID_INDEX_1].ecx&  bit_AVX)
> 148     {
> 149       /* Reset the AVX bit in case OSXSAVE is disabled.  */
> 150       if ((__cpu_features.cpuid[COMMON_CPUID_INDEX_1].ecx&  bit_OSXSAVE) != 0
> 151&&  ({ unsigned int xcrlow;
> 152                 unsigned int xcrhigh;
> 153                 asm ("xgetbv"
> 154                      : "=a" (xcrlow), "=d" (xcrhigh) : "c" (0));
> 155                 (xcrlow&  6) == 6; }))
> 156         __cpu_features.feature[index_YMM_Usable] |= bit_YMM_Usable;
> 157     }
>
>
> And when I ran a little silly C program (attached) to probe the CPUID flags, I got:
>   /root/test-xsave
> SSE3 CMOV
> AVX  XSAVE
> Trying XGETBV
> Illegal instruction (core dumped)
>
> Which would imply that the OSXSAVE is not set (but Fedora Core 17 64-bit PV guest
> still crashes on that machine) - which means that in multi-lib the bit_YMM_Usable
> is _not_ set. But then looking at the sources I see:
>
> # ifdef USE_AS_STRCASECMP_L
> 102 ENTRY(__strcasecmp)
> 103         .type   __strcasecmp, @gnu_indirect_function
> 104         cmpl    $0, __cpu_features+KIND_OFFSET(%rip)
> 105         jne     1f
> 106         call    __init_cpu_features
> 107 1:
> 108 #  ifdef HAVE_AVX_SUPPORT
> 109         leaq    __strcasecmp_avx(%rip), %rax
> 110         testl   $bit_AVX, __cpu_features+CPUID_OFFSET+index_AVX(%rip)
> 111         jnz     2f
> 112 #  endif
>
> which would imply that the AVX bit is sampled here instead of the
> YMM one.
[ ... ]
I think Uli's position is that this code only uses AVX encodings, but 
not the YMM registers and thus the right check is for AVX.

That doesn't make sense to me given the text under availability and 
support here:

http://software.intel.com/en-us/articles/introduction-to-intel-advanced-vector-extensions/

According to my reading AVX can only be used if the hardware supports 
AVX *and* the OS supports XSAVE.    The only weasel language is "To use 
the Intel AVX extensions reliably in most settings ..."  Which Uli might 
be relying upon for his position.

Ironically, the code in init-arch used to look like:

  if (__cpu_features.cpuid[COMMON_CPUID_INDEX_1].ecx & bit_AVX)
     {
       /* Reset the AVX bit in case OSXSAVE is disabled.  */
       if ((__cpu_features.cpuid[COMMON_CPUID_INDEX_1].ecx & 
bit_OSXSAVE) == 0
           || ({ unsigned int xcrlow;
               unsigned int xcrhigh;
               asm ("xgetbv"
                    : "=a" (xcrlow), "=d" (xcrhigh) : "c" (0));
               (xcrlow & 6) != 6; }))
         __cpu_features.cpuid[COMMON_CPUID_INDEX_1].ecx &= ~bit_AVX;
     }


Which I think would have done the right thing.   Uli changed it to the 
form you quoted just 2 hours after installing the version I quoted.

If i'm going to make the claim Uli is wrong, some clarification from 
Intel would be appreciated.


jeff