linux-kernel - Re: [RFC PATCH] x86/head_64.S: remove redundant check that kernel address is 2M aligned

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Message-ID: <CAMzpN2gR2dpyDU1RDKtq0oeOqcFuJiVtMkN9TkZSUGQrsjGEJQ@mail.gmail.com>
Date:	Fri, 22 Jan 2016 20:42:03 -0500
From:	Brian Gerst <brgerst@...il.com>
To:	Alexander Kuleshov <kuleshovmail@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H . Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...e.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] x86/head_64.S: remove redundant check that kernel
 address is 2M aligned

On Fri, Jan 22, 2016 at 1:13 PM, Alexander Kuleshov
<kuleshovmail@...il.com> wrote:
> We check that the base address of the kernel is 2M aligned in
> the arch/x86/kernel/head_65.S right after jump to the decompressed
> kernel. But we already have a check in the decompress_kernel()
> which validates that kernel location is MIN_KERNEL_ALIGN aligned
> which is 2M too for x86_64.
>
> Signed-off-by: Alexander Kuleshov <kuleshovmail@...il.com>
> ---
>  arch/x86/kernel/head_64.S | 6 ------
>  1 file changed, 6 deletions(-)
>
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index ffdc0e8..4967cba 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -75,12 +75,6 @@ startup_64:
>         leaq    _text(%rip), %rbp
>         subq    $_text - __START_KERNEL_map, %rbp
>
> -       /* Is the address not 2M aligned? */
> -       movq    %rbp, %rax
> -       andl    $~PMD_PAGE_MASK, %eax
> -       testl   %eax, %eax
> -       jnz     bad_address
> -
>         /*
>          * Is the address too large?
>          */

I think we still need to do the check, in case we came from a 64-bit
bootloader that directly jumped to startup_64.  However, this check
can be simplified to:

    testl $~PMD_PAGE_MASK, %ebp
    jnz bad_address

--
Brian Gerst