Re: [PATCH v7 0/5] xen/arm: Don't switch TTBR while the MMU is on

[Julien Grall <julien@xxxxxxx>]

Hi,

On 16/04/2023 15:32, Julien Grall wrote:
 > 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 temporarily 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 and will be handled in a follow-up
> series.
>
> 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-v7
>
> Cheers,
>
> Julien Grall (5):
>    xen/arm32: head: Widen the use of the temporary mapping
>    xen/arm64: Rework the memory layout
>    xen/arm64: mm: Introduce helpers to prepare/enable/disable the
>      identity mapping
>    xen/arm64: mm: Rework switch_ttbr()
>    xen/arm64: smpboot: Directly switch to the runtime page-tables
This series is now fully committed.

Thanks for the help to review it!

Cheers,

--
Julien Grall