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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id A3813C678DB for ; Fri, 24 Feb 2023 15:15:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230159AbjBXPP2 (ORCPT ); Fri, 24 Feb 2023 10:15:28 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35934 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230283AbjBXPPQ (ORCPT ); Fri, 24 Feb 2023 10:15:16 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 1FA7254A0A; Fri, 24 Feb 2023 07:14:44 -0800 (PST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 335B21042; Fri, 24 Feb 2023 07:15:01 -0800 (PST) Received: from [10.57.16.161] (unknown [10.57.16.161]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 4AE7D3FB75; Fri, 24 Feb 2023 07:14:15 -0800 (PST) Message-ID: <59a61ddc-4a96-c846-c12b-0d1e3789683c@arm.com> Date: Fri, 24 Feb 2023 15:14:11 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:102.0) Gecko/20100101 Thunderbird/102.7.2 Subject: Re: [PATCH 2/5] rust: device: Add a minimal RawDevice trait Content-Language: en-GB To: Asahi Lina , Greg Kroah-Hartman Cc: Miguel Ojeda , Alex Gaynor , Wedson Almeida Filho , Boqun Feng , Gary Guo , =?UTF-8?Q?Bj=c3=b6rn_Roy_Baron?= , Will Deacon , Joerg Roedel , Hector Martin , Sven Peter , Arnd Bergmann , "Rafael J. Wysocki" , Alyssa Rosenzweig , Neal Gompa , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, asahi@lists.linux.dev References: <20230224-rust-iopt-rtkit-v1-0-49ced3391295@asahilina.net> <20230224-rust-iopt-rtkit-v1-2-49ced3391295@asahilina.net> <87de1cd0-a57a-3391-a4b8-599ee0307491@asahilina.net> From: Robin Murphy In-Reply-To: <87de1cd0-a57a-3391-a4b8-599ee0307491@asahilina.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023-02-24 14:48, Asahi Lina wrote: > > > On 2023/02/24 23:32, Robin Murphy wrote: >> On 2023-02-24 14:11, Greg Kroah-Hartman wrote: >>> Thanks for the detailed rust explainations, I'd like to just highlight >>> one thing: >>> >>> On Fri, Feb 24, 2023 at 10:15:12PM +0900, Asahi Lina wrote: >>>> On 24/02/2023 20.23, Greg Kroah-Hartman wrote: >>>>> And again, why are bindings needed for a "raw" struct device at all? >>>>> Shouldn't the bus-specific wrappings work better? >>>> >>>> Because lots of kernel subsystems need to be able to accept "any" device >>>> and don't care about the bus! That's what this is for. >>> >>> That's great, but: >>> >>>> All the bus >>>> wrappers would implement this so they can be used as an argument for all >>>> those subsystems (plus a generic one when you just need to pass around >>>> an actual owned generic reference and no longer need bus-specific >>>> operations - you can materialize that out of a RawDevice impl, which is >>>> when get_device() would be called). That's why I'm introducing this now, >>>> because both io_pgtable and rtkit need to take `struct device` pointers >>>> on the C side so we need some "generic struct device" view on the >>>> Rust side. >>> >>> In looking at both ftkit and io_pgtable, those seem to be good examples >>> of how "not to use a struct device", so trying to make safe bindings >>> from Rust to these frameworks is very ironic :) >>> >>> rtkit takes a struct device pointer and then never increments it, >>> despite saving it off, which is unsafe.  It then only uses it to print >>> out messages if things go wrong (or right in some cases), which is odd. >>> So it can get away from using a device pointer entirely, except for the >>> devm_apple_rtkit_init() call, which I doubt you want to call from rust >>> code, right? >>> >>> for io_pgtable, that's a bit messier, you want to pass in a device that >>> io_pgtable treats as a "device" but again, it is NEVER properly >>> reference counted, AND, it is only needed to try to figure out the bus >>> operations that dma memory should be allocated from for this device.  So >>> what would be better to save off there would be a pointer to the bus, >>> which is constant and soon will be read-only so there are no lifetime >>> rules needed at all (see the major struct bus_type changes going into >>> 6.3-rc1 that will enable that to happen). >> >> FWIW the DMA API *has* to know which specific device it's operating >> with, since the relevant properties can and do vary even between >> different devices within a single bus_type (e.g. DMA masks). >> >> In the case of io-pgtable at least, there's no explicit refcounting >> since the struct device must be the one representing the physical >> platform/PCI/etc. device consuming the pagetable, so if that were to >> disappear from underneath its driver while the pagetable is still in >> use, things would already have gone very very wrong indeed :) > > There's no terribly good way to encode this relationship in safe Rust as > far as I know. So although it might be "obvious" (and I think my driver > can never violate it as it is currently designed), this means the Rust > abstraction will have to take the device reference if the C side does > not, because safe rust abstractions have to actually make these bugs > impossible and nothing stops a Rust driver from, say, stashing an > io_pgtable reference into a global and letting the device go away. If someone did that, then simply holding a struct device reference wouldn't guarantee much, since it only prevents the pointer itself from becoming invalid - it still doesn't say any of the data *in* the structure is still valid and "safe" for what a DMA API call might do with it. At the very least you'd probably have to somehow also guarantee that the device has a driver bound (which is the closest thing to a general indication of valid DMA ops across all architectures) and block it from unbinding for the lifetime of the reference, but that would then mean a simple driver which expects to tear down its io-pgtable from its .remove callback could never be unbound due to the circular dependency :/ Cheers, Robin.