Re: [PATCH 27/36] xen/arch: add coloring support for Xen

[Julien Grall <julien@xxxxxxx>]

Hi,

On 04/03/2022 19:47, Julien Grall wrote:
> On 04/03/2022 17:46, Marco Solieri wrote:
> > From: Luca Miccio <lucmiccio@xxxxxxxxx>
> >
> > Introduce a new implementation of setup_pagetables that uses coloring
> > logic in order to isolate Xen code using its color selection.
> > Page tables construction is essentially copied, except for the xenmap
> > table, where coloring logic is needed.  Given the absence of a contiguous
> > physical mapping, pointers to next level tables need to be manually
> > calculated.
> The implementation of setup_pagetables() is not compliant to the Arm 
> Arm. And I have plan to completely get rid of it.
> The main part that is not compliant is switch_ttbr() because it keeps 
> the MMU on. We should switch the MMU off, update the TTBR and then 
> switch on the MMU. This implies that we need an identity mapping of the 
> part of Xen that will run with MMU off.
> I understand that rebuilding the page-tables and therefore switching the 
> TTBR will be necessary for cache coloring. So before any new use, I 
> would like the implementation of switch_ttbr() to be fixed.
> What we will need to do is find space in the virtual layout that also 
> match a physical address. With that in place, we could use the mapping 
> to switch between TTBR.
I have posted an early RFC [1] to reshuffle the memory layout on Arm so 
we have space to for the identity mapping. I have also reworked 
switch_ttbr() to turn off/on the MMU before/after updating the TTBR.
The series should work on arm64. The arm32 effort requires a bit more 
effort as we have less virtual space.
I haven't killed setup_pagetables() yet so you have a base to write the 
cache coloring version. There may be also some tweak necessary for cache 
coloring (e.g. flush the instruction cache).
Cheers,

[1] https://lore.kernel.org/xen-devel/20220309112048.17377-1-julien@xxxxxxx/

--
Julien Grall