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

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Date:   Wed, 8 Mar 2017 17:45:05 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Bhupesh Sharma <bhsharma@...hat.com>
Cc:     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,
        bp@...e.de
Subject: Re: [PATCH 1/2] x86/efi: Correct a tiny mistake in code comment

On 03/08/17 at 02:30pm, Bhupesh Sharma wrote:
> Hi Dave,
> 
> On Wed, Mar 8, 2017 at 1:48 PM, Dave Young <dyoung@...hat.com> wrote:
> > On 03/08/17 at 03:47pm, Baoquan He wrote:
> >> - * We allocate runtime services regions bottom-up, starting from -4G, i.e.
> >> + * We allocate runtime services regions top-down, starting from -4G, i.e.
> >
> > Baoquan, I think original bottom-up is right, it is just considering
> > -68G as up, see the x86_64 mm.txt. We regard vmalloc as higher address
> > although from mathematics view it is lower then positive addresses.
> 
> I think you have a valid point, but I think the -4G convention is
> probably too confusing to read and may lead to issues when we use this
> for future feature addition as well. It would be more useful to use
> the macros similar to the MODULES_{} addresses we use currently in
> 'arch/x86/include/asm/pgtable_64_types.h':
> 
> #define MODULES_VADDR    (__START_KERNEL_map + KERNEL_IMAGE_SIZE)
> #define MODULES_END      _AC(0xffffffffff000000, UL)
> #define MODULES_LEN   (MODULES_END - MODULES_VADDR)
> 
> May be we can use the following convention for the EFI_VA_{} addresses
> as per 'http://lxr.free-electrons.com/source/Documentation/x86/x86_64/mm.txt#L19':
> 
> #define EFI_VA_START    _AC(0xfffffffeffffffff, UL)
> #define EFI_VA_END    _AC(0xffffffef00000000, UL)

Isn't it like this:

#define EFI_VA_START    _AC(0xffffffff00000000, UL)
#define EFI_VA_END    _AC(0xffffffef00000000, UL)

Just make them be equal to value which computer stores -4G and -68G?

You can see in arch/x86/platform/efi/efi_64.c, it's using efi_va minus
size directly, here size should be 4K, page aligned. 

	efi_va -= size;

Above formula has considered the open interval attribute of
EFI_VA_START. Making EFI_VA_START be 0xfffffffeffffffff could be wrong.

Right?