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

[Xenia Ragiadakou <burzalodowa@gmail.com>]

Hi Julien

On 10/22/22 18:04, Julien Grall wrote:
> From: Julien Grall <jgrall@amazon.com>
> 
> 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.

> 
> Cheers,
> 
> Julien Grall (12):
>    xen/arm: Clean-up the memory layout
>    xen/arm32: head: Jump to the runtime mapping in enable_mmu()
>    xen/arm32: head: Introduce an helper to flush the TLBs
>    xen/arm32: head: Remove restriction where to load Xen
>    xen/arm32: head: Widen the use of the temporary mapping
>    xen/arm: Enable use of dump_pt_walk() early during boot
>    xen/arm64: Rework the memory layout
>    xen/arm: mm: Allow xen_pt_update() to work with the current root table
>    xen/arm: mm: Allow dump_hyp_walk() to work on the current root table
>    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
> 
>   xen/arch/arm/arm32/head.S           | 253 ++++++++++++++++++----------
>   xen/arch/arm/arm32/smpboot.c        |   4 +
>   xen/arch/arm/arm64/Makefile         |   1 +
>   xen/arch/arm/arm64/head.S           |  86 +++++-----
>   xen/arch/arm/arm64/mm.c             | 160 ++++++++++++++++++
>   xen/arch/arm/arm64/smpboot.c        |  15 +-
>   xen/arch/arm/domain_page.c          |   9 +
>   xen/arch/arm/include/asm/arm32/mm.h |   4 +
>   xen/arch/arm/include/asm/arm64/mm.h |  12 ++
>   xen/arch/arm/include/asm/config.h   |  63 +++++--
>   xen/arch/arm/include/asm/mm.h       |   2 +
>   xen/arch/arm/include/asm/setup.h    |  11 ++
>   xen/arch/arm/include/asm/smp.h      |   1 +
>   xen/arch/arm/mm.c                   | 105 +++++++-----
>   xen/arch/arm/smpboot.c              |   1 +
>   15 files changed, 536 insertions(+), 191 deletions(-)
>   create mode 100644 xen/arch/arm/arm64/mm.c
> 

-- 
Xenia