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Date: Mon, 23 Jan 2017 13:16:40 -0800
From: Yu-cheng Yu <yu-cheng.yu@intel.com>
To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: fenghua.yu@intel.com, dvlasenk@redhat.com, peterz@infradead.org,
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Subject: Re: [tip:x86/urgent] x86/fpu: Set the xcomp_bv when we fake up a
 XSAVES area
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On Mon, Jan 23, 2017 at 01:10:20PM -0800, Dave Hansen wrote:
> The code is:
> 
> > void fpstate_init(union fpregs_state *state)
> > {
> >         if (!static_cpu_has(X86_FEATURE_FPU)) {
> >                 fpstate_init_soft(&state->soft);
> >                 return;
> >         }
> > 
> >         memset(state, 0, fpu_kernel_xstate_size);
> > 
> >         /*
> >          * XRSTORS requires that this bit is set in xcomp_bv, or
> >          * it will #GP. Make sure it is replaced after the memset().
> >          */
> >         if (static_cpu_has(X86_FEATURE_XSAVES))
> >                 state->xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT;
> 
> That seems to set it unconditionally.  What am I missing?

The fix I am proposing is...

	state->xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT |
				       xfeatures_mask;