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

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Date:   Thu, 09 Feb 2017 19:51:29 -0500
From:   Rik van Riel <riel@...hat.com>
To:     Borislav Petkov <bp@...e.de>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, luto@...nel.org,
        dave.hansen@...ux.intel.com, yu-cheng.yu@...el.com, hpa@...or.com
Subject: Re: [PATCH v2] x86/fpu: copy MXCSR & MXCSR_FLAGS with SSE/YMM state

On Fri, 2017-02-10 at 01:02 +0100, Borislav Petkov wrote:
> On Thu, Feb 09, 2017 at 06:43:47PM -0500, Rik van Riel wrote:
> > On Skylake CPUs I noticed that XRSTOR is unable to deal with xsave
> > areas
> > created by copyout_from_xsaves if the xstate has only SSE/YMM
> > state, but
> > no FP state. That is, xfeatures had XFEATURE_MASK_SSE set, but not
> > XFEATURE_MASK_FP.
> > 
> > The reason is that part of the SSE/YMM state lives in the MXCSR and
> > MXCSR_FLAGS fields of the FP area.
> > 
> > Ensure that whenever we copy SSE or YMM state around, the MXCSR and
> > MXCSR_FLAGS fields are also copied around.
> > 
> > Signed-off-by: Rik van Riel <riel@...hat.com>
> > ---
> >  arch/x86/kernel/fpu/xstate.c | 44
> > ++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 44 insertions(+)
> 
> ...
> 
> > @@ -987,6 +1004,13 @@ int copy_xstate_to_kernel(void *kbuf, struct
> > xregs_state *xsave, unsigned int of
> >  
> >  	}
> >  
> > +	if (xfeatures_need_mxcsr_copy(header.xfeatures)) {
> > +		offset = offsetof(struct fxregs_state, mxcsr);
> > +		size = sizeof(u64); // copy mxcsr & mxcsr_flags
> 
> 				    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> We don't do // comments, do we?
> 
> And side-line comments are always impairing the readability of the
> code
> unless it is a struct's members or asm or so ...

Good point. OTOH, I don't really want to add an extra line
to each of these blocks of code, either...

Ingo, how would you like me to do these comments?

Or should I have a magic #define with comment somewhere,
like this?

/* Copy both mxcsr & mxcsr_flags */
#define MXCSR_AND_FLAGS_SIZE sizeof(u64)