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Date: Mon, 24 Dec 2012 15:56:35 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Yinghai Lu <yinghai@kernel.org>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v7 00/27] x86, boot, 64bit: Add support for loading ramdisk
 and bzImage above 4G
References: <1355814959-10573-1-git-send-email-yinghai@kernel.org> <20121222024225.GG3468@phenom.dumpdata.com> <eee81bb9-f11b-4ff8-8a84-72f122766fd8@email.android.com> <20121222132419.GB2936@phenom.dumpdata.com> <CAE9FiQV5QqONiTdnHzsjVRTbnEfyHEE3s=-=+a49F2a5se7K+A@mail.gmail.com> <6214ad82-5b7e-47f2-a9c1-98972bdee517@email.android.com> <CAE9FiQV-7_wz1jCZh3qDGt6Wq_JN6b1oAyjj3+goCPFnYZ+q9Q@mail.gmail.com>
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On 12/23/2012 10:00 AM, Yinghai Lu wrote:
> On Sun, Dec 23, 2012 at 6:33 AM, H. Peter Anvin <hpa@zytor.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