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[34.140.183.197]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48de0aaabb8sm4646155f8f.15.2026.10.13.02.38.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 13 Oct 2026 02:38:04 -0700 (PDT) Date: Tue, 13 Oct 2026 10:37:59 +0100 From: Vincent Donnefort To: Sumit Garg Cc: Will Deacon , brendan.jackman@linux.dev, catalin.marinas@arm.com, rppt@kernel.org, akpm@linux-foundation.org, sudeep.holla@kernel.org, jenswi@kernel.org, robh@kernel.org, mark.rutland@arm.com, ardb@kernel.org, thierry.reding@kernel.org, david@kernel.org, danielmentz@google.com, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, op-tee@lists.trustedfirmware.org, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 0/8] arm64: Unmap FF-A lent memory from direct map Message-ID: References: <20260921110050.3977591-1-vdonnefort@google.com> Precedence: bulk X-Mailing-List: devicetree@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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(). So the only case it would work would be when a reserved memory has set "linux,cma-default". -- Vincent > > 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