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

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Date:   Wed, 8 Mar 2017 17:09:55 +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
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:
> >> EFI allocate runtime services regions down from EFI_VA_START, -4G.
> >> It should be top-down handling.
> >>
> >> Signed-off-by: Baoquan He <bhe@...hat.com>
> >> ---
> >>  arch/x86/platform/efi/efi_64.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> >> index a4695da..6cbf9e0 100644
> >> --- a/arch/x86/platform/efi/efi_64.c
> >> +++ b/arch/x86/platform/efi/efi_64.c
> >> @@ -47,7 +47,7 @@
> >>  #include <asm/pgalloc.h>
> >>
> >>  /*
> >> - * 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)
> 
> which is less confusing to read in my opinion.
> 
> @Baoquan: Please share your views.

Yes, it looks better. I can repost with this change. Thanks.