Re: [PATCH early-RFC 4/5] xen/arm: mm: Rework switch_ttbr()

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 15 Mar 2022, Julien Grall wrote:
> On 14/03/2022 23:48, Stefano Stabellini wrote:
> > > > - we save the current mapping
> > > > - update it with the Xen 1:1
> > > > - switch_ttbr
> > > > - remove Xen 1:1
> > > > - restore mapping
> > > > 
> > > > It should work, right? Basically, a mapping conflict shouldn't be an
> > > > issue given that the mapping has only to live long enough to call
> > > > switch_ttbr_id.
> > > 
> > > Today switch_ttbr() is called before we initialized most of the memory
> > > layout.
> > > So clashing with the VMAP and frametable is not a problem.
> > > 
> > > However, the identity mapping may also clash with the region used to map
> > > Xen.
> > > That said, technically, we are not able to handle Xen when its start
> > > address
> > > is in region 2MB + 4K to 4MB (Xen is loaded at a 4KB aligned address).
> > > 
> > > The trouble is some features (e.g. UBSAN, GCOV) can generate Xen image
> > > over
> > > 2MB. IOW, the range where Xen cannot be loaded will increase.
> > > 
> > > This is an issue because AFAIK, there is no away to tell GRUB "You can't
> > > load
> > > Xen at this region". But even if there were one, I feel this restriction
> > > is
> > > sort of random.
> > > 
> > > I already wrote a patch to get rid of the restriction. The code is not too
> > > bad
> > > (we only need an extra indirection). But I haven't sent it yet because it
> > > is
> > > less critical with the re-shuffling of the memory layout.
> > 
> > Interesting! I am curious: how did you manage to do it?
> 
> When the identity mapping is clashing with Xen runtime address, I am creating
> a temporary mapping for Xen at a different fixed address.
> 
> Once the MMU is turned on, we can jump to the temporary mapping. After that we
> are safe to remove the identity mapping and create the runtime Xen mapping.
> The last step is to jump on the runtime mapping and then remove the temporary
> mapping.

Cool! I was guessing something along those lines.