linux-kernel - Re: Edited kexec_load(2) [kexec_file

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Date:	Wed, 28 Jan 2015 17:25:26 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Scot Doyle <lkml14@...tdoyle.com>
Cc:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
	Kexec Mailing List <kexec@...ts.infradead.org>,
	Andy Lutomirski <luto@...capital.net>,
	Dave Young <dyoung@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: Edited kexec_load(2) [kexec_file_load()] man page for review

On Wed, Jan 28, 2015 at 10:10:59PM +0000, Scot Doyle wrote:
> On Wed, 28 Jan 2015, Vivek Goyal wrote:
> > On Wed, Jan 28, 2015 at 09:14:03PM +0000, Scot Doyle wrote:
> > > On Wed, 28 Jan 2015, Vivek Goyal wrote:
> > > > On Wed, Jan 28, 2015 at 04:49:34PM +0100, Michael Kerrisk (man-pages) wrote:
> > > > > Hello Vivek,
> > > > > 
> > > > > >> I've made various adjustments to the page in the light of your comments
> > > > > >> above. Thanks!
> > > > > >
> > > > > > Thank you for following it up and improving kexec man page.
> > > > > 
> > > > > You're welcome. So, by now, I've made quite a lot of changes
> > > > > (including adding a number of cases under ERRORS). I think the revised
> > > > > kexec_load/kexec_file_load page is pretty much ready to go, but would
> > > > > you be willing to give the text below a check over first?
> > > > > 
> > > > 
> > > > Hi Michael,
> > > > 
> > > > I had a quick look and it looks good to me.
> > > > 
> > > > Thanks
> > > > Vivek
> > > 
> > > When I tested, kexec_file_load required CONFIG_RELOCATABLE. Is the same 
> > > true for kexec_load? Would it make sense to note this in the man pages 
> > > along with the need for CONFIG_KEXEC_FILE, etc? Or as an error message?
> > 
> > Hmm.., I can't see an explicity dependency between RELOCATABLE and
> > KEXEC. Both KEXEC and KEXEC_FILE should be able to load a kernel
> > even if it had RELOCATABLE=n.
> > 
> > Just that kernel will run from the address it has been built for.
> > 
> > Thanks
> > Vivek
> 
> Confusing, right? kexec_file_load returns -ENOEXEC and dmesg says 
> "kexec-bzImage64: XLF_CAN_BE_LOADED_ABOVE_4G is not set." which leads to
> arch/x86/boot/header.S line 396:
> 
> #if defined(CONFIG_RELOCATABLE) && defined(CONFIG_X86_64)  
>    /* kernel/boot_param/ramdisk could be loaded above 4g */
> # define XLF1 XLF_CAN_BE_LOADED_ABOVE_4G
> #else
> # define XLF1 0
> #endif

Ah, this one. Actually generic kexec file loading implementation does not
impose this restriction. It is the image specific loader part which
decides what kind of bzImage it can load.

Current implementation (kexec-bzimage64.c), is only supporting loading
bzImages which are 64bit and can be loaded above 4G. This simplifies
the implementation of loader.

But there is nothing which prevents one from implementing other image
loaders.

So instead of saying that kexec_file_load() depends on CONFIG_RELOCATABLE,
it might be better to say in man page that currently this system call
supports only loading a bzImage which is 64bit and which can be loaded
above 4G too.

Thanks
Vivek
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