On Tue, 13 Oct 2026 at 10:37:59 +0100, Vincent Donnefort wrote:
On Mon, Oct 12, 2026 at 06:46:12PM +0530, Sumit Garg wrote:
On Mon, 05 Oct 2026 at 12:02:55 +0100, Vincent Donnefort wrote:
Sorry, not sure why Sumit's email was dropped...
The problem was with my mutt config, it should be fixed now.
On Mon, Oct 05, 2026 at 11:48:00AM +0100, Vincent Donnefort wrote:
On Thu, Oct 01, 2026 at 06:04:06PM +0530, Sumit Garg wrote:
On Tue, 29 Sep 2026 at 12:33:07 +0100, Will Deacon wrote:
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.
Thanks.
> 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.
The main problem I see with this series is it tries to move from the generic CMA allocator to a reserved FF-A CMA pool which again comes from the DT. We already support reserved pool allocations via "no-map" which are discovered dynamically from OP-TEE.
I can't think of a way around a DT declaration. We need to know what region is PTE-level before the direct map is installed and of_reserved_mem is very handy for that purpose. And as this PTE-level mapping is costly we certainly only want to enable it for platforms that absolutely need it.
I am rather thinking of a way to turn the default CMA allocator to be PTE-level mapped. Since the overall idea for this dynamic protected buffers was to reuse memory efficiently, otherwise the reserved buffer approach is already there to use as "no-map" via DT.
When the default CMA allocator is setup, it is already too late (dma_contiguous_reserve() is called after map_mem()). I am not sure this can be reordered easily due to the dependency on dma_limits_init().
Yeah I saw that constraint too, let's leave it for future extension then although having this reordering dependency fixed will be ideal.
So the only case it would work would be when a reserved memory has set "linux,cma-default".
Let's rather just turn DT based default CMA pool to support PTE-level mapping and keep it configurable via Kconfig or cmdline or DT.
-Sumit
If we can have "cma_unmap" cmdline option alongside "cma" then it can turn the existing default CMA allocator to support unmapping buffers. Not requiring a DT change at all, although we can have the DT property for the default pool too if platforms want to opt for.
If we can rather support a generic CMA like allocator for arm64 which can support unmapped allocations then I am all for it. These platform specific reserved pools isn't a scalable solution.
What do you mean by generic CMA allocator here? An extension to shared-dma-pool? Or having the CMA automatically doing the unmap/map on cma_alloc() cma_release()?
Right now I have created an API for the unmapping/remapping part. It adds a burden on the caller, but the alternative solution is introducing post-alloc pre-dealloc callbacks in the CMA, which looked a bit overkilled to me when the users of that "ffa-pool" are only optee and the nvidia video dma-buf heap.
I am not too worried about where the actual implementation lives but looks like CMA driver to be the better place. I think use-cases like guest_memfd proposed as a user for __GFP_UNMAPPED on arm64 will also be looking for this feature.
-Sumit
You could allocate the DMABufs from a CMA region that has no linear alias.
Allocation from generic CMA pool is already the case here:
tee_shm_alloc_dma_mem() -> dma_alloc_pages()
Do you mean it's just fine to unmap buffers allocated from generic CMA region since it doesn't have any 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.
I agree with you here, what I am rather trying to find if on arm64 there is any possibilty to have a generic allocator for unmapped pages. That's the use-case here as well as what Brendan't patch-set was trying to address.
> 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.
Sure, sounds like you are eluding to that the generic MM flag __GFP_UNMAPPED being proposed can't be supported on arm64, right?
-Sumit
-- Vincent