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 0FA15C77B75 for ; Fri, 19 May 2023 17:10:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Content-Type: Content-Transfer-Encoding:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:From:References:Cc:To:Subject: MIME-Version:Date:Message-ID:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=BP3o28CpHbjB1oyPdreWDkREXjuHxa3OVmtcEDmlHBM=; b=Pao/sw8fpIse6d 6cr+l/w7n6GunZp2WJRCbBFnSA/qlbLU5kysCD25ybgCRiWoyX36XywwdvHjBBRAHveHvZGSd0y7J pYwsIZLrHH6hPdjI+OQothABOmPFHjyGqnl2aTGpfO7wwQFO/1MPMGkQqnbomn0IheJ3XKGpmMz0s aw1c80XcnSFWZ5dJ+Xp+vr8TA/HTQTH5HYjn7UsKOXf+qELX5Jq5hyF3/3L2+TxZ4u25gtQW+FN8r ifIWdck9kWkANcWdofJvnqkhA8sgTD9Ru6gAuTAgoyWd6xihRhKsYRrISBoCcb4D4qIO7eXUJSeiv wRCN/C+bJ2e1v9Spur9g==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.96 #2 (Red Hat Linux)) id 1q03c8-00GowF-0D; Fri, 19 May 2023 17:10:00 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.96 #2 (Red Hat Linux)) id 1q03c4-00Gour-0J for linux-arm-kernel@lists.infradead.org; Fri, 19 May 2023 17:09:58 +0000 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 E4D9C1FB; Fri, 19 May 2023 10:10:37 -0700 (PDT) Received: from [10.1.196.40] (e121345-lin.cambridge.arm.com [10.1.196.40]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 9653B3F59C; Fri, 19 May 2023 10:09:50 -0700 (PDT) Message-ID: <30a91384-157c-0192-443c-12c835ad3b35@arm.com> Date: Fri, 19 May 2023 18:09:45 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [PATCH v4 13/15] iommu/dma: Force bouncing if the size is not cacheline-aligned To: Catalin Marinas Cc: Linus Torvalds , Arnd Bergmann , Christoph Hellwig , Greg Kroah-Hartman , Will Deacon , Marc Zyngier , Andrew Morton , Herbert Xu , Ard Biesheuvel , Isaac Manjarres , Saravana Kannan , Alasdair Kergon , Daniel Vetter , Joerg Roedel , Mark Brown , Mike Snitzer , "Rafael J. Wysocki" , linux-mm@kvack.org, iommu@lists.linux.dev, linux-arm-kernel@lists.infradead.org References: <20230518173403.1150549-1-catalin.marinas@arm.com> <20230518173403.1150549-14-catalin.marinas@arm.com> <9dc3e036-75cd-debf-7093-177ef6c7a3ae@arm.com> Content-Language: en-GB From: Robin Murphy In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230519_100956_233018_83F029EA X-CRM114-Status: GOOD ( 36.59 ) 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: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 19/05/2023 3:02 pm, Catalin Marinas wrote: > On Fri, May 19, 2023 at 01:29:38PM +0100, Robin Murphy wrote: >> On 2023-05-18 18:34, Catalin Marinas wrote: >>> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c >>> index 7a9f0b0bddbd..ab1c1681c06e 100644 >>> --- a/drivers/iommu/dma-iommu.c >>> +++ b/drivers/iommu/dma-iommu.c >>> @@ -956,7 +956,7 @@ static void iommu_dma_sync_sg_for_cpu(struct device *dev, >>> struct scatterlist *sg; >>> int i; >>> - if (dev_use_swiotlb(dev)) >>> + if (dev_use_swiotlb(dev) || sg_is_dma_bounced(sgl)) >>> for_each_sg(sgl, sg, nelems, i) >>> iommu_dma_sync_single_for_cpu(dev, sg_dma_address(sg), >>> sg->length, dir); >>> @@ -972,7 +972,7 @@ static void iommu_dma_sync_sg_for_device(struct device *dev, >>> struct scatterlist *sg; >>> int i; >>> - if (dev_use_swiotlb(dev)) >>> + if (dev_use_swiotlb(dev) || sg_is_dma_bounced(sgl)) >>> for_each_sg(sgl, sg, nelems, i) >>> iommu_dma_sync_single_for_device(dev, >>> sg_dma_address(sg), >>> @@ -998,7 +998,8 @@ static dma_addr_t iommu_dma_map_page(struct device *dev, struct page *page, >>> * If both the physical buffer start address and size are >>> * page aligned, we don't need to use a bounce page. >>> */ >>> - if (dev_use_swiotlb(dev) && iova_offset(iovad, phys | size)) { >>> + if ((dev_use_swiotlb(dev) && iova_offset(iovad, phys | size)) || >>> + dma_kmalloc_needs_bounce(dev, size, dir)) { >>> void *padding_start; >>> size_t padding_size, aligned_size; >>> @@ -1210,7 +1211,21 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg, >>> goto out; >>> } >>> - if (dev_use_swiotlb(dev)) >>> + /* >>> + * If kmalloc() buffers are not DMA-safe for this device and >>> + * direction, check the individual lengths in the sg list. If one of >>> + * the buffers is deemed unsafe, follow the iommu_dma_map_sg_swiotlb() >>> + * path for potential bouncing. >>> + */ >>> + if (!dma_kmalloc_safe(dev, dir)) { >>> + for_each_sg(sg, s, nents, i) >>> + if (!dma_kmalloc_size_aligned(s->length)) { >> >> Just to remind myself, we're not checking s->offset on the grounds that if >> anyone wants to DMA into an unaligned part of a larger allocation that >> remains at their own risk, is that right? > > Right. That's the case currently as well and those users that were > relying on ARCH_KMALLOC_MINALIGN for this have either been migrated to > ARCH_DMA_MINALIGN in this series or the logic rewritten (as in the > crypto code). OK, I did manage to summon a vague memory of this being discussed before, which at least stopped me asking "Should we be checking..." - perhaps a comment on dma_kmalloc_safe() to help remember that reasoning might not go amiss? >> Do we care about the (probably theoretical) case where someone might build a >> scatterlist for multiple small allocations such that ones which happen to be >> adjacent might get combined into a single segment of apparently "safe" >> length but still at "unsafe" alignment? > > I'd say that's theoretical only. One could write such code but normally > you'd go for an array rather than relying on the randomness of the > kmalloc pointers to figure out adjacent objects. It also only works if > the individual struct size is exactly one of the kmalloc cache sizes, so > not generic enough. FWIW I was imagining something like sg_alloc_table_from_pages() but at a smaller scale, queueing up some list/array of, say, 32-byte buffers into a scatterlist to submit as a single DMA job. I'm not aware that such a thing exists though, and I'm inclined to agree that it probably is sufficiently unrealistic to be concerned about. As usual I just want to feel comfortable that we've explored all the possibilities :) >>> + sg_dma_mark_bounced(sg); >> >> I'd prefer to have iommu_dma_map_sg_swiotlb() mark the segments, since >> that's in charge of the actual bouncing. Then we can fold the alignment >> check into dev_use_swiotlb() (with the dev_is_untrusted() condition taking >> priority), and sync/unmap can simply rely on sg_is_dma_bounced() alone. > > With this patch we only set the SG_DMA_BOUNCED on the first element of > the sglist. Do you want to set this flag only on individual elements > being bounced? It makes some sense in principle but the > iommu_dma_unmap_sg() path would need to scan the list again to decide > whether to go the swiotlb path. > > If we keep the SG_DMA_BOUNCED flag only on the first element, I can > change it to your suggestion, assuming I understood it. Indeed that should be fine - sync_sg/unmap_sg always have to be given the same arguments which were passed to map_sg (and note that in the normal case, the DMA address/length will often end up concatenated entirely into the first element), so while we still have the two distinct flows internally, I don't think there's any issue with only tagging the head of the list to steer between them. Of course if it then works out to be trivial enough to tag *all* the segments for good measure, there should be no harm in that either - at the moment the flag is destined to have more of a "this might be bounced, so needs checking" meaning than "this definitely is bounced" either way. Cheers, Robin. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel