From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 335D6372AB6; Thu, 20 Nov 2025 08:06:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763626000; cv=none; b=mPHwMi7kFIx3eH5JDrOB0WVa1xEmKw/Ml/NX7FkeMC8LD6S6MTEUp2RmziW7630BeCNjX2WfB3XU5tVzh3SMdeussUb9/TKrXxT59Jtkn9S2UtEtueBTxBVy1c0DLCH+a0kE4LW+X44X+LfxyK/OJO1rGnD0DLS5G98n2hMxDNQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763626000; c=relaxed/simple; bh=mxT/rSlaqP+txsmWmS9/PssKwzGp7j3BhlaZuVwXZis=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=LKJVZSCXFLtQZaqK+/XQ1aLWr5XYa4X0huCO3MjuwXwLPyE9ewJERBVkY0CRkMdwlvYcHdxkz19TzHZt8lAIgCHYGobNFBMZAr1VYWCmwhdfo73mu1V6nyp9XYCPGN43TF0b4uTnIwRI9I6xs/IM0FOwubA6n/Z5T1kvEtPaxCU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Jyoe7Oe5; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Jyoe7Oe5" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 124CEC4CEF1; Thu, 20 Nov 2025 08:06:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1763625999; bh=mxT/rSlaqP+txsmWmS9/PssKwzGp7j3BhlaZuVwXZis=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Jyoe7Oe5fQPJ/c3G0Y3Le2MuwLeYij5ETlxaQsfHH0IeCeAp4SV8sma9/p4puBfJd uSJLTrxHP3QFvgERIjsj/1tqk+X/okArVSEm8ZMhuoWrUm+nw49VGAhsvO+/F7roqB earNWu4MD42Xdka7K6xqmgczk8orzPbhqOIDxQD+/aKHXhBiOGA4RplKw1T/I2rQ6a ev9a2LBRLzTP1F+YYtsiwBQ2gNaHKtotSqd6c5JXDYnlHf0a974bbsQ8w3T/BC46cr IlYGcGcRRMayKQVYgRTSLxtDvYEu5tk34YIf2hUPYoKt2aPFZV53WId4koKue3AXLS Eq5b6O0Zpw2Lw== Date: Thu, 20 Nov 2025 10:06:35 +0200 From: Leon Romanovsky To: Christian =?iso-8859-1?Q?K=F6nig?= Cc: Jason Gunthorpe , Bjorn Helgaas , Logan Gunthorpe , Jens Axboe , Robin Murphy , Joerg Roedel , Will Deacon , Marek Szyprowski , Andrew Morton , Jonathan Corbet , Sumit Semwal , Kees Cook , "Gustavo A. R. Silva" , Ankit Agrawal , Yishai Hadas , Shameer Kolothum , Kevin Tian , Alex Williamson , Krishnakant Jaju , Matt Ochs , linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, iommu@lists.linux.dev, linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, kvm@vger.kernel.org, linux-hardening@vger.kernel.org, Alex Mastro , Nicolin Chen Subject: Re: [Linaro-mm-sig] [PATCH v8 06/11] dma-buf: provide phys_vec to scatter-gather mapping routine Message-ID: <20251120080635.GT18335@unreal> References: <20251111-dmabuf-vfio-v8-0-fd9aa5df478f@nvidia.com> <20251111-dmabuf-vfio-v8-6-fd9aa5df478f@nvidia.com> <8a11b605-6ac7-48ac-8f27-22df7072e4ad@amd.com> <20251119132511.GK17968@ziepe.ca> <69436b2a-108d-4a5a-8025-c94348b74db6@amd.com> <20251119193114.GP17968@ziepe.ca> <20251120074137.GR18335@unreal> <209499e2-6a06-4291-ad4c-77230926c665@amd.com> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <209499e2-6a06-4291-ad4c-77230926c665@amd.com> On Thu, Nov 20, 2025 at 08:54:37AM +0100, Christian König wrote: > On 11/20/25 08:41, Leon Romanovsky wrote: > > On Thu, Nov 20, 2025 at 08:08:27AM +0100, Christian König wrote: > >> On 11/19/25 20:31, Jason Gunthorpe wrote: > >>> On Wed, Nov 19, 2025 at 02:42:18PM +0100, Christian König wrote: > >>> > >>>>>>> + case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE: > >>>>>>> + dma->state = kzalloc(sizeof(*dma->state), GFP_KERNEL); > >>>>>>> + if (!dma->state) { > >>>>>>> + ret = -ENOMEM; > >>>>>>> + goto err_free_dma; > >>>>>>> + } > >>>>>>> + > >>>>>>> + dma_iova_try_alloc(attach->dev, dma->state, 0, size); > >>>>>> > >>>>>> Oh, that is a clear no-go for the core DMA-buf code. > >>>>>> > >>>>>> It's intentionally up to the exporter how to create the DMA > >>>>>> addresses the importer can work with. > >>>>> > >>>>> I can't fully understand this remark? > >>>> > >>>> The exporter should be able to decide if it actually wants to use > >>>> P2P when the transfer has to go through the host bridge (e.g. when > >>>> IOMMU/bridge routing bits are enabled). > >>> > >>> Sure, but this is a simplified helper for exporters that don't have > >>> choices where the memory comes from. > >> > >> That is extremely questionable as justification to put that in common DMA-buf code. > >> > >>> I fully expet to see changes to this to support more use cases, > >>> including the one above. We should do those changes along with users > >>> making use of them so we can evaluate what works best. > >> > >> Yeah, exactly that's my concern. > >> > >>>> But only take that as Acked-by, I would need at least a day (or > >>>> week) of free time to wrap my head around all the technical details > >>>> again. And that is something I won't have before January or even > >>>> later. > >>> > >>> Sure, it is alot, and I think DRM community in general should come up > >>> to speed on the new DMA API and how we are pushing to see P2P work > >>> within Linux. > >>> > >>> So thanks, we can take the Acked-by and progress here. Interested > >>> parties can pick it up from this point when time allows. > >> > >> Wait a second. After sleeping a night over it I think my initial take that we really should not put that into common DMA-buf code seems to hold true. > >> > >> This is the use case for VFIO, but I absolutely want to avoid other drivers from re-using this code until be have more experience with that. > >> > >> So to move forward I now strongly think we should keep that in VFIO until somebody else comes along and needs that helper. > > > > It was put in VFIO at the beginning, but Christoph objected to it, > > because that will require exporting symbol for pci_p2pdma_map_type(). > > which was universally agreed as not good idea. > > Yeah, that is exactly what I object here :) > > We can have the helper in DMA-buf *if* pci_p2pdma_map_type() is called by drivers or at least accessible. That's what I pointed out in the other mail before as well. > > The exporter must be able to make decisions based on if the transaction would go over the host bridge or not. > > Background is that in a lot of use cases you rather want to move the backing store into system memory instead of keeping it in local memory if the driver doesn't have direct access over a common upstream bridge. > > Currently drivers decide that based on if IOMMU is enabled or not (and a few other quirks), but essentially you absolutely want a function which gives this information to exporters. For the VFIO use case it doesn't matter because you can't switch the BAR for system memory. > > To unblock you, please add a big fat comment in the kerneldoc of the mapping explaining this and that it might be necessary for exporters to call pci_p2pdma_map_type() as well. Thanks, What do you think about it? diff --git a/drivers/dma-buf/dma-buf-mapping.c b/drivers/dma-buf/dma-buf-mapping.c index a69bb73db86d..05ec84a0157b 100644 --- a/drivers/dma-buf/dma-buf-mapping.c +++ b/drivers/dma-buf/dma-buf-mapping.c @@ -84,6 +84,11 @@ struct dma_buf_dma { * PAGE_SIZE aligned. * * A mapping must be unmapped by using dma_buf_free_sgt(). + * + * NOTE: While this function is intended for DMA-buf importers, it is critical + * that the DMA-buf exporter is capable of performing peer-to-peer (P2P) DMA + * directly between PCI devices, without routing transactions through the host + * bridge. */ struct sg_table *dma_buf_phys_vec_to_sgt(struct dma_buf_attachment *attach, struct p2pdma_provider *provider, (END) > > Regards, > Christian. > > > > > https://lore.kernel.org/all/aPYrEroyWVOvAu-5@infradead.org/ > > > > Thanks > > > >> > >> Regards, > >> Christian. > >> > >>> > >>> We can also have a mini-community call to give a summary/etc on these > >>> topics. > >>> > >>> Thanks, > >>> Jason > >> >