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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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?

Ian.


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