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Date:   Fri, 26 Jul 2019 09:43:27 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     John Hubbard <jhubbard@nvidia.com>
cc:     "H. Peter Anvin" <hpa@zytor.com>, john.hubbard@gmail.com,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/1] x86/boot: clear some fields explicitly
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On Thu, 25 Jul 2019, John Hubbard wrote:
> On 7/25/19 3:28 PM, H. Peter Anvin wrote:
> > On 7/25/19 3:03 PM, Thomas Gleixner wrote:
> >> On Thu, 25 Jul 2019, hpa@zytor.com wrote:
> >>> On July 25, 2019 2:48:30 PM PDT, Thomas Gleixner <tglx@linutronix.de> wrote:
> >>>>
> >>>> But seriously I think it's not completely insane what they are doing
> >>>> and the table based approach is definitely more readable and maintainable
> >>>> than the existing stuff.
> >>>
> >>> Doing this table based does seem like a good idea.
> >>
> >> The question is whether we use a 'toclear' table or a 'preserve' table. I'd
> >> argue that the 'preserve' approach is saner.
> >>
> > 
> > I agree.
> > 
> 
> OK, I can polish up something and post it, if you can help me with one more
> quick question: how did you want "to preserve" to work?
> 
> a) copy out fields to preserve, memset the area to zero, copy back preserved
> fields? This seems like it would have the same gcc warnings as we have now,
> due to the requirement to memset a range of a struct... 

Use the same trick I used for the toclear variant.

#define PRESERVE(m)			\
	{				\
		.start = offsetof(m),	\
		.len   = sizeof(m),	\
	}

sanitize_boot_params(bp, scratch)
{
	char *p1 = bp, *p2 = scratch;

	preserve[] = {
		PRESERVE(member1),
		...
		PRESERVE(memberN),
	};

	for_each_preserve(pr)
		memcpy(p2 + pr->start, p1 + pr->start, pr->len)
	memcpy(bp, scratch, ...);
}


Thanks,

	tglx