Date: Sat, 17 Dec 2016 19:46:45 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Larry Finger <Larry.Finger@lwfinger.net>
Cc: LKML <linux-kernel@vger.kernel.org>, Vivek Goyal <vgoyal@redhat.com>
Subject: Re: What is the function of arch/x86/purgatory/purgatory.c?
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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().