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

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Date:	Sat, 23 Jan 2016 12:37:56 +0600
From:	Alexander Kuleshov <kuleshovmail@...il.com>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Alexander Kuleshov <kuleshovmail@...il.com>,
	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

Hello Brian,

On 01-22-16, Brian Gerst wrote:
> >
> > -       /* 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

Ah, ok, in this way we can't trust a bootloader. I just thought that
64-bit entry point is startup_64 from arch/x86/boot/compressed/head_64.S

Thank you.