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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-07-15 at 12:07 +0100, Julien Grall wrote:
> 
> On 15/07/14 12:03, Julien Grall wrote:
> >
> >
> > 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.
> 
> Hrm... I misread the documentation for this part.
> 
> "The image must be placed at the specified offset (currently 0x80000)
> from the start of the system RAM and called there. The start of the
> system RAM must be aligned to 2MB."
> 

Regardless of this being more flexible in what load addresses we accept
is relatively easy, has no impact after boot and the code is already
written.

Ian.


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