linux-kernel - Re: What is the function of arch/x86/purgatory/purgatory.c?

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Date:   Sat, 17 Dec 2016 13:56:37 -0600
From:   Larry Finger <Larry.Finger@...inger.net>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Vivek Goyal <vgoyal@...hat.com>
Subject: Re: What is the function of arch/x86/purgatory/purgatory.c?

On 12/17/2016 01:46 PM, Al Viro wrote:
> On Sat, Dec 17, 2016 at 11:52:05AM -0600, Larry Finger wrote:
>
>> Upon examination of the routine, I can see that if purgatory() should be
>> static, then none of the code here will ever be accessed by any part of the
>> kernel. Is there some bit of magic that is above my understanding, or is
>> this a useless bit of code that has been forgotten and should be removed?
>
> I don't know what is and what is not above your understanding, but grepping
> in that area (grep -w purgatory arch/x86/purgatory/*) does catch this:
> arch/x86/purgatory/setup-x86_64.S:      call purgatory
> which is hardly magic - looks like a function call.  Looking into that
> file shows
> purgatory_start:
>         .code64
>
>         /* Load a gdt so I know what the segment registers are */
>         lgdt    gdt(%rip)
>
>         /* load the data segments */
>         movl    $0x18, %eax     /* data segment */
>         movl    %eax, %ds
>         movl    %eax, %es
>         movl    %eax, %ss
>         movl    %eax, %fs
>         movl    %eax, %gs
>
>         /* Setup a stack */
>         leaq    lstack_end(%rip), %rsp
>
>         /* Call the C code */
>         call purgatory
>         jmp     entry64
>
> which pretty much confirms that - it's called from purgatory_start().

Thanks for the explanation.

Larry