linux-kernel - Re: [tip:x86/urgent] x86/fpu: Set the xcomp

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Date:   Mon, 23 Jan 2017 13:28:20 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     fenghua.yu@...el.com, dvlasenk@...hat.com, peterz@...radead.org,
        oleg@...hat.com, mingo@...nel.org, linux-kernel@...r.kernel.org,
        brgerst@...il.com, luto@...nel.org, bp@...en8.de,
        jpoimboe@...hat.com, haokexin@...il.com, hpa@...or.com,
        quentin.casasnovas@...cle.com, tglx@...utronix.de,
        torvalds@...ux-foundation.org, riel@...hat.com,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86/fpu: Set the xcomp_bv when we fake up a
 XSAVES area

On 01/23/2017 01:16 PM, Yu-cheng Yu wrote:
> 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;

Ahh, that makes sense.  That does indeed look like a bug in fpstate_init().