linux-kernel - Re: [PATCH v2] x86/fpu: copy MXCSR & MXCSR

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Date:   Thu, 09 Feb 2017 20:00:11 -0500
From:   Rik van Riel <riel@...hat.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, luto@...nel.org,
        dave.hansen@...ux.intel.com, bp@...e.de, hpa@...or.com
Subject: Re: [PATCH v2] x86/fpu: copy MXCSR & MXCSR_FLAGS with SSE/YMM state

On Thu, 2017-02-09 at 16:45 -0800, Yu-cheng Yu wrote:
> On Thu, Feb 09, 2017 at 06:43:47PM -0500, Rik van Riel wrote:
> >  /*
> > + * Weird legacy quirk: SSE and YMM states store information in the
> > + * MXCSR and MXCSR_FLAGS fields of the FP area. That means if the
> > FP
> > + * area is marked as unused in the xfeatures header, we need to
> > copy
> > + * MXCSR and MXCSR_FLAGS if either SSE or YMM are in use.
> > + */
> > +static inline bool xfeatures_need_mxcsr_copy(u64 xfeatures)
> > +{
> > +	if (!(xfeatures & (XFEATURE_MASK_SSE|XFEATURE_MASK_YMM)))
> > +		return 0;
> > +
> > +	if (xfeatures & XFEATURE_MASK_FP)
> > +		return 0;
> > +
> > +	return 1;
> > +}
> 
> Would you consider separating MXCSR & MXCSR_FLAGS from
> XFEATURE_MASK_FP.  Here we assume if both xstate[0] and xstate[1]
> are being copied, then there is no need to copy MXCSR/MXCSR_FLAGS
> again.  What if only xstate[0] is copied and MXCSR/MXCSR_FLAGS
> is invalid?

Surely then the CPU would ignore the contents of
MXCSR/MXCSR_FLAGS because the SSE and YMM bits in
the xfeatures header are clear?

What am I missing?