linux-kernel - Re: [PATCH 1/2] x86/efi: Correct a tiny mistake in code comment

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Message-ID: <CACi5LpOXgCfysVdSnaCW=VPGp4VVJm9EKssvGK0T4wZ6ew7cvw@mail.gmail.com>
Date:   Thu, 9 Mar 2017 01:35:00 +0530
From:   Bhupesh Sharma <bhsharma@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Baoquan He <bhe@...hat.com>, Dave Young <dyoung@...hat.com>,
        linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
        thgarnie@...gle.com, Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH 1/2] x86/efi: Correct a tiny mistake in code comment

On Wed, Mar 8, 2017 at 3:05 PM, Borislav Petkov <bp@...en8.de> wrote:
> On Wed, Mar 08, 2017 at 05:09:55PM +0800, Baoquan He wrote:
>> Yes, it looks better. I can repost with this change. Thanks.
>
> No it doesn't:
>
> #define EFI_VA_START     ( -4 * (_AC(1, UL) << 30))
> #define EFI_VA_END       (-68 * (_AC(1, UL) << 30))
>
> That's -4G (the shift by 30) and -68G, respectively.
>
>> > #define EFI_VA_START    _AC(0xfffffffeffffffff, UL)
>> > #define EFI_VA_END    _AC(0xffffffef00000000, UL)
>
> That is something which I need to type into a calculator first.

Right, my point was that this -4G convention stands out, as compared
to the rest of the addressing convention used throughout
'arch/x86/include/asm/pgtable_64_types.h'.

For e.g.:

#define __VMALLOC_BASE    _AC(0xffffc90000000000, UL)
#define __VMEMMAP_BASE    _AC(0xffffea0000000000, UL)
..
#define MODULES_END      _AC(0xffffffffff000000, UL)
..
and so on.

Also it seems inconsistent to the convention used in
'Documentation/x86/x86_64/mm.txt'

As you noted in one of your other comments in this thread, this causes
a confusion as to whether the EFI_VA_END and EFI_VA_START macros need
to be swapped or the comments elsewhere in the x86 code which use
these MACROS are incorrect.

However, I think it is more a matter of code readability and each of
these styles have their own advantages.

I would be happy to cook up a patch to have uniformity in the
addressing conventions used across pgtable_64_types.h (if it is
required at all), but it would be a different topic and not related to
this patch.

Regards,
Bhupesh