linux-kernel - Re: [PATCH v7 00/27] x86, boot, 64bit: Add support for loading ramdisk and bzImage above 4G

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Message-ID: <50D8EBB3.5040408@zytor.com>
Date:	Mon, 24 Dec 2012 15:56:35 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 00/27] x86, boot, 64bit: Add support for loading ramdisk
 and bzImage above 4G

On 12/23/2012 10:00 AM, Yinghai Lu wrote:
> On Sun, Dec 23, 2012 at 6:33 AM, H. Peter Anvin <hpa@...or.com> wrote:
>> Explanation please?
>>
> 
> you have following change in the patch
> 
>         /* Finally jump to run C code and to be on real kernel address
>          * Since we are running on identity-mapped space we have to jump
>          * to the full 64bit address, this is only possible as indirect
>          * jump.  In addition we need to ensure %cs is set so we make this
> -        * a far return.
> +        * a far jump.
>          */
> -       movq    initial_code(%rip),%rax
>         pushq   $0              # fake return address to stop unwinder
> -       pushq   $__KERNEL_CS    # set correct cs
> -       pushq   %rax            # target address in negative space
> -       lretq
> +       /* gas 2.22 is buggy and mis-assembles ljmpq */
> +       rex64 ljmp *initial_code(%rip)
> 
>  #ifdef CONFIG_HOTPLUG_CPU
>  /*
> 
> remove that change, AMD systems works again.
> 

That is not an explanation.

	-hpa


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