Re: [RFC PATCH 0/1] static-memory: allow skipping the cache flush

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 18/09/2026 22:23, Jan Setje-Eilers wrote:
> On 9/17/26 04:06, Grall, Julien wrote:
> > A few years ago, Arm introduced FEAT_FWB to rework how stage-2 + stage-1
> > memory attributes works. Currently in Xen, we are using the "old"
> > approach where the used attributes is the strongest of the two. For
> > instance, the guest has the MMU off, then the access will bypass the
> > cache. With FEAT_FWB, you can force the memory attributes to be
> > cacheable even when the MMU is off.
> >
> > I believe FEAT_FWB should solve the problem you have (assume your HW
> > supports it). In term of effort, it is mostly updating the stage-2
> > memory attributes and then getting rid of some cache operations (include
> > set/way emulation) when the feature is enabled.
>   I found that as well, but at least some of my hardware is from just
> before that shows up.
Thanks for clarification. I would have though most of such HW are pretty 
much gone :).
I am not entirely sure what's the best way forward for HW not supporting 
FEAT_FWB. With your approach, I am concerned we may be missing some 
cases after boot, so we could potentially skip the flush if the pages 
have not been touched for Xen (you said the pages are not scrubbed for 
static memory). I'd like some opinions from Bertrand, Michal and Stefano 
on this.
Cheers,

--
Julien Grall