linux-kernel - Re: [Qemu-devel] [RFC/PoC PATCH 1/3] i386: set initrd_max to 4G

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Message-ID: <20181112061940.GA61749@gmail.com>
Date:   Mon, 12 Nov 2018 07:19:40 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Li Zhijian <zhijianx.li@...el.com>,
        Juergen Gross <jgross@...e.com>,
        Li Zhijian <lizhijian@...fujitsu.com>,
        Peter Maydell <peter.maydell@...aro.org>, x86@...nel.org,
        bp@...en8.de, mingo@...hat.com, tglx@...utronix.de,
        QEMU Developers <qemu-devel@...gnu.org>,
        Philip Li <philip.li@...el.com>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [Qemu-devel] [RFC/PoC PATCH 1/3] i386: set initrd_max to 4G - 1
 to allow up to 4G initrd


* H. Peter Anvin <hpa@...or.com> wrote:

> > Such an extended header could use a more modern (self-extending) ABI as 
> > well.
> 
> Yes, although I don't really think it is as much of an issue as it seems at
> this point.
> 
> The limit comes from having used a one-byte jump instruction at the beginning;
> however, these days that limit is functionally walled.
> 
> It is of course possible to address this if it should become necessary,
> however, the current protocol has lasted for 23 years so far and we haven't
> run out yet, even with occasional missteps. As such, I don't think we are in a
> huge hurry to address this particular aspect.

Agreed, fair enough!

> In part as a result of this exchange I have spent some time thinking 
> about the boot protocol and its dependencies, and there is, in fact, a 
> much more serious problem that needs to be addressed: it is not 
> currently possible in a forward-compatible way to map all data areas 
> that may be occupied by bootloader-provided data. The kernel proper has 
> an advantage here, in that the kernel will by definition always be the 
> "owner of the protocol" (anything the kernel doesn't know how to map 
> won't be used by the kernel anyway), but it really isn't a good 
> situation. So I'm currently trying to think up a way to make that 
> possible.

I might be a bit dense early in the morning, but could you elaborate? 
What do you mean by mapping all data areas?

Thanks,

	Ingo