linux-kernel - Re: [PATCH] x86, boot: Allow 64bit EFI kernel to be loaded above 4G

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Date:	Sun, 22 Feb 2015 21:18:01 +0800
From:	Baoquan He <bhe@...hat.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86, boot: Allow 64bit EFI kernel to be loaded above 4G

On 02/21/15 at 10:49am, Baoquan He wrote:
> On 02/20/15 at 03:53pm, Yinghai Lu wrote:
> > Then you are not setting  the ident mapping correctly.
> > 
> > you should make sure add extra ident mapping for the new [output,
> > output+output_len - 1].
> > bootloader only cover old [output, output+output_len - 1]
> > 
> > and you should check if the mapping is present before add new one,
> > otherwise will overrite
> > the one from 64bit bootloader like kexec-tools or grub2-x86_64 etc.
> > 
> > You could use kernel_ident_mapping_init() from arch/x86/mm/init_64.c
> > --- may need to cut and paste or split and include to
> > arch/x86/boot/compressed/misc.c
> > also you need to find some pages for alloc_pgt_page.
> 
> At the beginning I did it just as you said, add IDT table and $PF
> handler. Get page fault address and built ident mapping around it when
> reload kernel above 4G. In this case 3 more pages are enough if kernel
                                      ~~
                                       typo, it should be 4

> is put to another 512G and cross the boundary of 512G.
> kernel_ident_mapping_init code can be borrowed and need be adjusted a
> little bit. This works as expected, but a GPF reported and reboot to
> BIOS. That's why I made a simple debug patch as I pasted before to
> filter unnecessary interference.
> 
> Since in arch/x86/boot/compressed/head_64.S 6 pages are used, 1 for pgd,
> 1 for pud, 4 for pmd, all of them cover 0~4G ident mapping. So I added 4
> more pages as pmd, then 0~8G are covered. Now I hardcoded the output of
> randomization of physical address as 5G, means kernel will be reloaded
> there and decompressed. With these ident mapping for this hard coded
> address should be correctly setted, still that GPF will happen. I
> borrowed a printf.c from arch/x86/boot and made it work for
> boot/compressed, can see in this case it will decompress successfully
> and jump into arch/x86/kernel/head_64.S, then it reboot there during a
> jump.
> 
> > 
> > Thanks
> > 
> > Yinghai
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