linux-kernel - Re: Does anything ever enter through startup_64 in head

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Message-ID: <24722.1481129227@warthog.procyon.org.uk>
Date:   Wed, 07 Dec 2016 16:47:07 +0000
From:   David Howells <dhowells@...hat.com>
To:     Matt Fleming <matt@...eblueprint.co.uk>
Cc:     dhowells@...hat.com, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Does anything ever enter through startup_64 in head_64.S?

Matt Fleming <matt@...eblueprint.co.uk> wrote:

> > Does anything ever enter the kernel through startup_64 in head_64.S?[*]  Do
> > all 64-bit mode entries always enter through one of the EFI entry points?
>  
> Which head_64.S? There are two ;-)
> 
> Assuming you mean startup_64 in boot/compressed/head_64.S, then the
> answer is "yes". 64-bit BIOS boot loaders will jump there.

Sorry, yes, that one.

> I'm fairly sure it's possible to boot that way on EFI too, you just
> lose some of the newer functionality that is dependent on the EFI boot
> stub. But I don't remember the last time I tried it.
> 
> In general, we've always tried to maintain backwards compat, but you
> don't get the new features without switching to the EFI entry points.
> 
> Whether or not any distros are still using the old 0x200 entry point
> for EFI is a good question, though.

I think I have to assume that there might be.

David