From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 19980CC6B03 for ; Tue, 13 Oct 2026 09:56:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=Q7fj9Vp+tJj7vJqMJATzo0vyG2yGkhzhjEgsT39DDVI=; b=vbbW4m6nz+DKU8r7rff2mbNlO5 u7pX3r+Jl/Hj6R4qNa60E5v++rnunTM2TYzx9E/muDD2JeTMop6UPw/iXjzgmTNKLCcfwZXWAl+2W w9lanE+GSkeLUFyf1vXgAAEXXyExNr6akEGvQEHeeVn9jfGlra6ZgIPyxvwAvqa6tN3pQRSVfA4e2 6OVmqDdwGjBqbvBXcEi8L/d+NtnkSV6l+hLh7/YzLyBHNzA3IYvncculIkrYMJn9yjB1xULP64rz5 KdeWOf+mprd04VktU3rc/srTOiYobd5JyVivM+WXywaTf0N5St2o61qKV/tmOqiKt7PFb82Gk/GLC IOXlijgw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1xGZFR-0000000CUE1-3YW5; Tue, 13 Oct 2026 09:56:41 +0000 Received: from tor.source.kernel.org ([2600:3c04:e001:324:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1xGZFQ-0000000CUDu-0u47 for linux-arm-kernel@lists.infradead.org; Tue, 13 Oct 2026 09:56:40 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 433E1600D1; Tue, 13 Oct 2026 09:56:39 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C55F31F000FF; Tue, 13 Oct 2026 09:56:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791885399; bh=Q7fj9Vp+tJj7vJqMJATzo0vyG2yGkhzhjEgsT39DDVI=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=FL6bAAK1wmdEJY0kUnZUcDtzMSWohJp9SOCaOF43QWDUhT0/iL+EQrzkosR+1+gbK n3CYnRul7jUIP5IfLrkv+P8pOPh2mpHNS0Uk7wGHOYoV0XS1R0aERG9phPvMAFz/m5 QcZdYbCJMmhUbrAflNX57QafmAdlmP34KxetYLnLS9rdkS7h4MXNYvwBtSTDHXg93c LXsP/R+zgGNL6f6XUxUQk4JWLfXvGkhLVMFwJUUaIpFMyzcLic/mA0L8NUYvPS8sHb 2QPEtRlQjY4pusEYwNfnlZ8LI5GQyhY4CKycn3vvy+P2KTQn0xXPvmjqDkcMg8QP74 qEjGgM633sPKg== Date: Tue, 13 Oct 2026 15:26:30 +0530 From: Sumit Garg To: Vincent Donnefort Cc: Sumit Garg , 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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 >