Re: [Xen-devel] [PATCH 2/5] xen: arm: Handle 4K aligned hypervisor load address.

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 15/07/14 10:13, Ian Campbell wrote:
> On Mon, 2014-07-14 at 23:33 +0100, Julien Grall wrote:
> > Hi Ian,
> >
> > On 14/07/14 17:39, Ian Campbell wrote:
> > > Currently the boot page tables map Xen at XEN_VIRT_START using a 2MB section
> > > mapping. This means that the bootloader must load Xen at a 2MB aligned address.
> > > Unfortunately this is not the case with UEFI on the Juno platform where Xen
> > > fails to boot. Furthermore the Linux boot protocol (which Xen claims to adhere
> > > to) does not have this restriction, therefore this is our bug and not the
> > > bootloader's.
> > >
> > > Fix this by adding third level pagetables to the boot time pagetables, allowing
> > > us to map a Xen which is aligned only to a 4K boundary. This only affects the
> > > boot time page tables since Xen will later relocate itself to a 2MB aligned
> > > address. Strictly speaking the non-boot processors could make use of this and
> > > use a section mapping, but it is simpler if all processors follow the same boot
> > > path.
> > OOI, did you think about the solution to copy Xen to a 2MB aligned
> > address before setup the early page table?
> I did but I was worried about clobbering something useful, like a boot
> module.
>
> > It would avoid to use third level page table during boot
> This isn't really that expensive, and it's marked __init so it goes away
> after boot.
>
> >   but will
> > introduce an extra copy of Xen (only for the boot processor).
> >
> > FYI, it's what Linux does.
> Interesting, how does it choose the address? Just by rounding down the
> loading address?
For ARM32 yes.

For ARM64, I don't find anything about relocating the Image. AFAIU, they 
use a field in the header to specify at which offset we need to load the 
kernel from the RAM base address.
On Xen, this field is set to 0, which means loading at any address. So I 
suspect for ARM64 we can change this offset and avoid to modify page 
table code.
Regards,

--
Julien Grall

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