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

On Tue, 2014-07-15 at 13:03 +0100, Julien Grall wrote:
> > 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.
> I stopped to count the number of patch I completely reworked after the 
> first version...

Fortunately no one is asking you to rework this patch and I'm perfectly
happy to do so myself.

> I think this is adding complexity in the assembly code and restriction 
> (see your panic) where the bootloader load Xen in the memory.

It removes restrictions and adds none (except that it currently
incorrectly rejects being loaded at exactly 2M right now, that's a
simple fix though).

I don't see it as being much more complex, it's essentially the same
pattern as level 1 and 2 further up the page table tree.

>  Even 
> though, the restriction where already there but hidden by the fact we 
> are using 2MB mapping.

You have a strange definition of "adding restrictions" then.

> This could be replaced by:
>     ARM32: adding a couple of assembly lines to relocate down to a 2MB 
> address.
>     ARM64: using the offset in the Image, unless if we released 
> bootloader is not able to correctly cope with it.

I'm not 100% certain that either of those are completely viable.

For 32-bit I don't think we know what we will overwrite by copying
ourselves down. Perhaps it would be ok, but why risk it.

For 64-bit at least the Juno firmware appears to load us at 0x80080000
irrespective of the value put in the text offset field. That's certainly
a bug in the firmware, but I can't see any reason not to make ourselves
more flexible here.

I've no idea what control the UEFI stub is going to have over load
address, but I'm pretty sure it will be preferable to avoid having to
relocate on that code path too.


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