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

On 05/10/2012 06:58 PM, Konrad Rzeszutek Wilk wrote:
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) ==
          || ({ 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.

Sadly no as it would have executed the xgetv instruction. Since the first
part of the boolean logic returns false.

<sigh>  And that is what I get from typing this while stopping at
lights and being in a hurry and doing this on a cellphone.
Please ignore what I said above - the earlier version would have worked correct.
No worries :-)

Had you not gotten involved, I never would have seen the xen-devel thread which then referred me to the appropriate Intel doc to confirm that your patch should fix the problem. Your input on this issue has been greatly appreciated.


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