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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 098A2C433FE for ; Mon, 14 Nov 2022 15:14:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237170AbiKNPOc (ORCPT ); Mon, 14 Nov 2022 10:14:32 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42160 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231609AbiKNPOa (ORCPT ); Mon, 14 Nov 2022 10:14:30 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 3A9601F2F7 for ; Mon, 14 Nov 2022 07:14:29 -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 1382623A; Mon, 14 Nov 2022 07:14:35 -0800 (PST) Received: from [10.57.70.90] (unknown [10.57.70.90]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 665E73F534; Mon, 14 Nov 2022 07:14:27 -0800 (PST) Message-ID: <1659929b-1372-cea6-5840-c58369a4252d@arm.com> Date: Mon, 14 Nov 2022 15:14:21 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:102.0) Gecko/20100101 Thunderbird/102.4.1 Subject: Re: [PATCH] Revert "arm64: dma: Drop cache invalidation from arch_dma_prep_coherent()" Content-Language: en-GB To: Will Deacon , Manivannan Sadhasivam Cc: catalin.marinas@arm.com, amit.pundir@linaro.org, andersson@kernel.org, quic_sibis@quicinc.com, sumit.semwal@linaro.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20221114110329.68413-1-manivannan.sadhasivam@linaro.org> <20221114141109.GG30263@willie-the-truck> From: Robin Murphy In-Reply-To: <20221114141109.GG30263@willie-the-truck> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022-11-14 14:11, Will Deacon wrote: > On Mon, Nov 14, 2022 at 04:33:29PM +0530, Manivannan Sadhasivam wrote: >> This reverts commit c44094eee32f32f175aadc0efcac449d99b1bbf7. >> >> As reported by Amit [1], dropping cache invalidation from >> arch_dma_prep_coherent() triggers a crash on the Qualcomm SM8250 platform >> (most probably on other Qcom platforms too). The reason is, Qcom >> qcom_q6v5_mss driver copies the firmware metadata and shares it with modem >> for validation. The modem has a secure block (XPU) that will trigger a >> whole system crash if the shared memory is accessed by the CPU while modem >> is poking at it. >> >> To avoid this issue, the qcom_q6v5_mss driver allocates a chunk of memory >> with no kernel mapping, vmap's it, copies the firmware metadata and >> unvmap's it. Finally the address is then shared with modem for metadata >> validation [2]. >> >> Now because of the removal of cache invalidation from >> arch_dma_prep_coherent(), there will be cache lines associated with this >> memory even after sharing with modem. So when the CPU accesses it, the XPU >> violation gets triggered. > > This last past is a non-sequitur: the buffer is no longer mapped on the CPU > side, so how would the CPU access it? Right, for the previous change to have made a difference the offending part of this buffer must be present in some cache somewhere *before* the DMA buffer allocation completes. Clearly that driver is completely broken though. If the DMA allocation came from a no-map carveout vma_dma_alloc_from_dev_coherent() then the vmap() shenanigans wouldn't work, so if it backed by struct pages then the whole dance is still pointless because *a cacheable linear mapping exists*, and it's just relying on the reduced chance that anything's going to re-fetch the linear map address after those pages have been allocated, exactly as I called out previously[1]. Robin. [1] https://lore.kernel.org/linux-arm-kernel/97fface8-e40e-072c-4335-c94094884e93@arm.com/ > As I just replied to Amit, we need more information about what this > "access" is and how it is being detected. > > Will