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

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Date:   Fri, 10 Feb 2017 09:00:54 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Rik van Riel <riel@...hat.com>
Cc:     Borislav Petkov <bp@...e.de>, 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


* Rik van Riel <riel@...hat.com> wrote:

> 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)

Yeah, that define would make it pretty clear what's going on. Please make it a bit 
more vebose:

  /* Copy both mxcsr & mxcsr_flags with a single u64 memcpy: */
  #define MXCSR_AND_FLAGS_SIZE sizeof(u64)

As for same-line comments, it can be the usual comment form:

		size = sizeof(u64); /* Copy mxcsr & mxcsr_flags */

But MXCSR_AND_FLAGS_SIZE is more expressive.

BTW., you can also use a separate comment line in such cases:

		/* Copy mxcsr & mxcsr_flags in one u64 step: */
		size = sizeof(u64);

... as readability is more important than brevity.

It's the C++ comment style that is frowned upon, as it looks weird in Linux kernel 
code.

Thanks,

	Ingo