From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bali.collaboradmins.com (bali.collaboradmins.com [148.251.105.195]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4405F339860; Fri, 19 Dec 2025 15:12:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=148.251.105.195 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766157122; cv=none; b=sUEB/ZIY5bImSGCh85pnSu6wq+Gradvshsv6R9vVyBlC6/iQqXC0yiMeAOq6JTyfzfyOByrC8XwrntONXonlrne+j46YP5GtJxJDNLClgb0BNLj2mtwof5rlCP7jSmsNLet4g7g9QgeNCko5sgEsstxY0elUMOV9urhs375eJ2s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766157122; c=relaxed/simple; bh=hbXC2AdsrfOcablKNwPdrwTw50dVWbz4YA+va0S+KXg=; h=Date:From:To:Cc:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=dk0r0ZATdH2u0Uv+EIMSrdoouFygDD1qJ7NxGSsvRNO6yqzbW5pV4WR1+3+m0BvqZltrdGJADMGGfMdbKtM+wphSacI6KHq5CFfzmhMnfzJ+IPLZtsdsbjvVyp8eAisikKrwvEFYvoIt+GcroFQOQH8O/WAhjyalcqK2wG342ss= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=collabora.com; spf=pass smtp.mailfrom=collabora.com; dkim=pass (2048-bit key) header.d=collabora.com header.i=@collabora.com header.b=fm8LUnUO; arc=none smtp.client-ip=148.251.105.195 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=collabora.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=collabora.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=collabora.com header.i=@collabora.com header.b="fm8LUnUO" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1766157118; bh=hbXC2AdsrfOcablKNwPdrwTw50dVWbz4YA+va0S+KXg=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=fm8LUnUON6zeA0olAxNc0JU+8h3LF7uwreCv3zmfhIqhq3cZIVfo691ye0odnLRjx ya8cG6pNcSmizToFufznzPsErbNG8+ZH6J8/ihp/tkBL8ElCiWasFSSGZU+do880P2 l1a+SXVcQtzwxTEpXy94YKF/UUw7Nqg2rxGb4ZHYKfxrB4W7nnP9lXxKoLyrArRHrq bLgVJFttqPGml0Gd21HOgdDgLOl7Lu7Na2gMtf3T6O8kkpDPcNusGaxq00jP1xhGMJ L5O9f+aakBx+R/WoniuXLojoh1DXotOx7RC24OshSCo1vfg4S4JlfryWWrXkkNUgPV xl+2BLkR5XZSQ== Received: from fedora (unknown [IPv6:2a01:e0a:2c:6930:d919:a6e:5ea1:8a9f]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (prime256v1) server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: bbrezillon) by bali.collaboradmins.com (Postfix) with ESMTPSA id 820E817E0CA3; Fri, 19 Dec 2025 16:11:57 +0100 (CET) Date: Fri, 19 Dec 2025 16:11:53 +0100 From: Boris Brezillon To: Alice Ryhl Cc: Jason Gunthorpe , Miguel Ojeda , Will Deacon , Daniel Almeida , Robin Murphy , Boqun Feng , Gary Guo , "=?UTF-8?B?QmrDtnJu?= Roy Baron" , Benno Lossin , Andreas Hindborg , Trevor Gross , Danilo Krummrich , Joerg Roedel , Lorenzo Stoakes , "Liam R. Howlett" , Asahi Lina , linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, iommu@lists.linux.dev, linux-mm@kvack.org Subject: Re: [PATCH v4] io: add io_pgtable abstraction Message-ID: <20251219161153.420d1c46@fedora> In-Reply-To: References: <20251219-io-pgtable-v4-1-68aaa7a40380@google.com> <20251219140557.GH31492@ziepe.ca> Organization: Collabora X-Mailer: Claws Mail 4.3.1 (GTK 3.24.51; x86_64-redhat-linux-gnu) Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Fri, 19 Dec 2025 14:38:51 +0000 Alice Ryhl wrote: > On Fri, Dec 19, 2025 at 10:05:57AM -0400, Jason Gunthorpe wrote: > > On Fri, Dec 19, 2025 at 10:50:52AM +0000, Alice Ryhl wrote: > > > +// For now, we do not provide the ability to flush the TLB via the built-in callback mechanism. > > > +// Instead, the `map_pages` function requires the caller to explicitly flush the TLB before the > > > +// pgtable is used to access the newly created range. > > > +// > > > +// This is done because the initial user of this abstraction may perform many calls to `map_pages` > > > +// in a single batched operation, and wishes to only flush the TLB once after performing the entire > > > +// batch of mappings. These callbacks would flush too often for that use-case. > > > +// > > > +// Support for flushing the TLB in these callbacks may be added in the future. > > > +static NOOP_FLUSH_OPS: bindings::iommu_flush_ops = bindings::iommu_flush_ops { > > > + tlb_flush_all: Some(rust_tlb_flush_all_noop), > > > + tlb_flush_walk: Some(rust_tlb_flush_walk_noop), > > > + tlb_add_page: None, > > > +}; > > > > This comment seems quite off.. > > > > Usually you don't flush on map, you flush on unmap. The TLB should be > > empty upon mapping and not need flushing - except for the rarer > > special cases of clearing the walk cache which cannot be detected any > > other way than using these callbacks. Doing a big flush on map to deal > > with the walk cache would be worse than implementing these callbacks. > > > > The flush on unmap, at least for ARM style invalidations, needs these > > callbacks because they provide required information. If the actual HW > > does not use an ARM style invalidation system then this page table > > code is not optimal for it. > > You should not assume that the way I worded something implies that the > GPU hardware does something weird. It's more likely that I just got > something wrong. > > It looks like panthor / tyr flush the range that was modified after both > map and unmap operations. There's actually a confusion between TLB invalidation and L1/L2 cache flush/invalidation. The things we can decide to flush/invalidate around map/unmap ops are L1/L2 caches. The TLB invalidate, we don't have direct control on: it happens as part of the LOCK+UNLOCK sequence, and no matter what you execute (map or unmap), you have to surround it with a LOCK/UNLOCK to provide support for atomic updates (GPU is blocked if anything accesses the locked range while an update is on-going). Robin, feel free to correct me if I'm wrong.