Re: [RFC v2 00/12] xen/arm: Don't switch TTBR while the MMU is on

[Julien Grall <julien@xxxxxxx>]

On 24/10/2022 16:39, Xenia Ragiadakou wrote:
> Hi Julien
Hi Xenia,

> On 10/22/22 18:04, Julien Grall wrote:
> > From: Julien Grall <jgrall@xxxxxxxxxx>
> >
> > Hi all,
> >
> > Currently, Xen on Arm will switch TTBR whilst the MMU is on. This is
> > similar to replacing existing mappings with new ones. So we need to
> > follow a break-before-make sequence.
> >
> > When switching the TTBR, we need to temporary disable the MMU
> > before updating the TTBR. This means the page-tables must contain an
> > identity mapping.
> >
> > The current memory layout is not very flexible and has an higher chance
> > to clash with the identity mapping.
> >
> > On Arm64, we have plenty of unused virtual address space Therefore, we 
> > can
> > simply reshuffle the layout to leave the first part of the virtual
> > address space empty.
> >
> > On Arm32, the virtual address space is already quite full. Even if we
> > find space, it would be necessary to have a dynamic layout. So a
> > different approach will be necessary. The chosen one is to have
> > a temporary mapping that will be used to jumped from the ID mapping
> > to the runtime mapping (or vice versa). The temporary mapping will
> > be overlapping with the domheap area as it should not be used when
> > switching on/off the MMU.
> >
> > The Arm32 part is not yet addressed in this version. The series is
> > sent as an early RFC to gather some feedback on the approach.
> >
> > After this series, most of Xen page-table code should be compliant
> > with the Arm Arm. The last two issues I am aware of are:
> >   - domheap: Mappings are replaced without using the Break-Before-Make
> >     approach.
> >   - The cache is not cleaned/invalidated when updating the page-tables
> >     with Data cache off (like during early boot).
> >
> > The long term plan is to get rid of boot_* page tables and then
> > directly use the runtime pages. This means for coloring, we will
> > need to build the pages in the relocated Xen rather than the current
> > Xen.
> >
> > For convience, I pushed a branch with everything applied:
> >
> > https://xenbits.xen.org/git-http/people/julieng/xen-unstable.git
> > branch boot-pt-rework-v2
> Are you sure that the branch has been pushed remotely? If yes, ignore.
I forgot to push it. It should now be pushed.

Cheers,

--
Julien Grall