RE: Proposal for Porting Xen to Armv8-R64 - DraftA

[Stefano Stabellini <sstabellini@kernel.org>]

On Mon, 7 Mar 2022, Wei Chen wrote:
> > > On 01/03/2022 07:51, Wei Chen wrote:
> > > >>> ### 1.2. Xen Challenges with PMSA Virtualization
> > > >>> Xen is PMSA unaware Type-1 Hypervisor, it will need modifications to
> > > run
> > > >>> with an MPU and host multiple guest OSes.
> > > >>>
> > > >>> - No MMU at EL2:
> > > >>>       - No EL2 Stage 1 address translation
> > > >>>           - Xen provides fixed ARM64 virtual memory layout as basis
> > of
> > > >> EL2
> > > >>>             stage 1 address translation, which is not applicable on
> > > MPU
> > > >> system,
> > > >>>             where there is no virtual addressing. As a result, any
> > > >> operation
> > > >>>             involving transition from PA to VA, like ioremap, needs
> > > >> modification
> > > >>>             on MPU system.
> > > >>>       - Xen's run-time addresses are the same as the link time
> > > addresses.
> > > >>>           - Enable PIC (position-independent code) on a real-time
> > > target
> > > >>>             processor probably very rare.
> > > >>
> > > >> Aside the assembly boot code and UEFI stub, Xen already runs at the
> > > same
> > > >> address as it was linked.
> > > >>
> > > >
> > > > But the difference is that, base on MMU, we can use the same link
> > > address
> > > > for all platforms. But on MPU system, we can't do it in the same way.
> > >
> > > I agree that we currently use the same link address for all the
> > > platforms. But this is also a problem when using MMU because EL2 has a
> > > single TTBR.
> > >
> > > At the moment we are switching page-tables with the MMU which is not
> > > safe. Instead we need to turn out the MMU off, switch page-tables and
> > > then turn on the MMU. This means we need to have an identity mapping of
> > > Xen in the page-tables. Assuming Xen is not relocated, the identity
> > > mapping may clash with Xen (or the rest of the virtual address map).
> > >
> > 
> > Is this the same reason we create a dummy reloc section for EFI loader?
> > 
> > > My initial idea was to enable PIC and update the relocation at boot
> > > time. But this is a bit cumbersome to do. So now I am looking to have a
> > > semi-dynamic virtual layout and find some place to relocate part of Xen
> > > to use for CPU bring-up.
> > >
> > > Anyway, my point is we possibly could look at PIC if that could allow
> > > generic Xen image.
> > >
> > 
> > I understand your concern. IMO, PIC is possible to do this, but obviously,
> > it's not a small amount of work. And I want to hear some suggestions from
> > Stefano, because he also has some solutions in previous thread.
> >
> 
> Can you have a look at the PIC discussion between Julien and me?
> I think we may need some inputs from your view.

If we have to have a build-time device tree anyway, we could
automatically generate the link address, together with other required
addresses. There would little benefit to do PIC if we have to have a
build-time device tree in any case.

On the other hand, if we could get rid of the build-time device tree
altogether, then yes doing PIC provides some benefits. It would allow us
to have single Xen binary working on multiple Cortex-R boards. However,
I don't think that would be important from a user perspective. People
will not install Ubuntu on a Cortex-R and apt-get xen.  The target is
embedded: users will know from the start the board they will target, so
it would not be a problem for them to build Xen for a specific board.
ImageBuilder (or something like it) will still be required to generate
boot scripts and boot info. In other words, although it would be
convenient to produce a generic binary, it is not a must-have feature
and I would consider it low-priority compared to others.