Re: [Xen-devel] [PATCH v3 2/6] xen: arm: Handle 4K aligned hypervisor load address.

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2014-07-21 at 15:38 +0200, Tim Deegan wrote:
> At 14:29 +0100 on 21 Jul (1405949358), Ian Campbell wrote:
> > On Mon, 2014-07-21 at 15:20 +0200, Tim Deegan wrote:
> > > At 13:59 +0100 on 21 Jul (1405947596), Ian Campbell wrote:
> > > > +virtphys_clash:
> > > > +        /* Identity map clashes with boot_third, which we cannot 
> > > > handle yet */
> > > > +        PRINT("Unable to build boot page tables - virt and phys 
> > > > addresses clash.\r\n")
> > > 
> > > Please tag this string "- Like so -\r\n" to match the other early output
> > > from the boot assembler.
> > 
> > OK. (I may do it on commit unless other comments necessitate a resend)
> 
> Sure.
> 
> > > Presumably we could get around this by making a 4k mapping when the 2M
> > > mapping would clash (at the cost of some extra code)?  
> > 
> > It might be workable.
> > 
> > We'd have to steal the slot from boot_third to point to 4K worth of 1:1
> > mapping instead of the expected 4K of virtual mapping and then put back
> > the virt mapping right after switching to the virtually mapped PC before
> > we touch anything mapped via that slot i.e. before touching anything
> > after _end_boot. Doing it at the same time as we establish the fixmap
> > and dtb mappings would do the trick, I think.
> > 
> > I'd previously concluded this wasn't possible, but don't recall why.
> > What am I missing this time?
> 
> We'd be in trouble if we used the slot that covers the boot_third
> table itself.  We could work around that, e.g. by using a linear PT to
> update the mapping, but it's getting a bit intricate.

That's not the issue I foresaw, but it is an issue.

A linear PT would be a problem if the virt address we picked clashed
with the load address.

We might be able to get away with mapping boot_third twice, one at it's
normal virt address and again at some address just after _end, then
using whichever one doesn't clash with the 1:1.

>   At that point
> I'd be inclined to look at ways to get up into C while still PIC and
> doing the pagetable build & trampoline from there.

Having played with this for grub purposes I don't really trust -fPIC
very much on ARM. Although that was trying to be doubly clever so maybe
it's OK for more traditional uses.

Ian.


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