Re: [PATCH v2 0/8] arm64: Unmap FF-A lent memory from direct map

[Will Deacon]

On Sat, Sep 26, 2026 at 05:43:08PM +0530, Sumit Garg wrote:
> Looks like you dropped me from your reply.

Huh, that's weird. If I hit reply-all ('g') in mutt, it moves the entire
CC list to To: and drops you. I suspect the same thing happened to Thierry
when he replied here:

https://lore.kernel.org/all/arJcN60nagFKXj2Q@orome/

I can't figure out why that's happening :/
I manually tried to fix the To: and Cc: lines in this message.

> On Tue, 22 Sep 2026 at 15:55:59 +0100, Will Deacon wrote:
> > I spoke to Brendan at LPC (?) last year but this doesn't really work
> > for arm64 because it relies on being able to unmap arbitrary parts of
> > the linear map, which isn't generally possible unless you force pte-level
> > mappings for everything, which is prohibitive for perf/power.
> 
> As per the cover letter it's about allocating pages that are not present
> in the direct map. This essentially fits the protected DMABufs use-case
> where we don't want any kernel mapping to exist at any time. Bufers
> allocated from protected DMABufs are only meant to be accessed by the
> TEE implementation or HW accelerators like in the secure media pipeline.

That sounds like it should probably build on top of this series, then.
You could allocate the DMABufs from a CMA region that has no linear alias.

> > Vincent's series tackles that by using a pool so that only that part of
> > memory requires the pte-level mappings in the linear map. That's the
> > whole point of it, so I don't think it makes sense to drop it in favour
> > of Brendan's approach (which doesn't work).
> > 
> 
> For protected DMABufs, we even don't require any pte-level mappings in
> linear map. The whole idea of mapping and unmapping is just a bottleneck
> for performance sensitive secure media pipeline use-cases. That's why if
> we can support __GFP_UNMAPPED with the memory allocator on arm64 would
> be the best fit for this use-case.

The point is that you will require pte-level mappings for the linear map
if you want to unmap from it at runtime on arm64. If you don't, then you
can end up needing to split a block mapping (e.g. pmd-level) when you
decide to unmap only part of it and there isn't a safe way to do that on
arm64 without transiently unmapping the entire block, which can lead to
fatal translation faults during that window.

> Now the question is why arm64 can't support __GFP_UNMAPPED similar to
> how Brendan is doing it for x86?

Because arm64 != x86? Presumably x86 doesn't have the strict
break-before-make requirements we have on arm64.

Will