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 3B66235F8B5; Wed, 19 Nov 2025 13:42:52 +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=1763559773; cv=none; b=KZlpzx5ui6FXh7WtSA1bUxX0K8fddmNpcHHa9MPCYOh6ZJ/iZzU4PnCnP1BEyQ//s8Qx0uuGVGchNTu6MXPeyHa++8enWcaSmDvXaPu1G4nH16N1o1d4LdRXDnJySFRJx7QcCGm7ZjZj8DwrL5kVGqKTfGhJaAYPuSL91/Smaug= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763559773; c=relaxed/simple; bh=P/9pmsmDZmkNlTyjbeOkKXUp9EgbKAdqcxRg8cTn9uY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=C31gd/5PwFYCFvlCizi2jRfegSPp5tTfQr4E9mL9R4sLYDY8DWy8UlC1TTDnbk4SeHoN+EGjZndaOmV2ta3jXQEeF6V/mkOqCpc/Pb6czac2sjTYGgr9zSxRTbQ1YJRzdO+zZupcxElCDLm2HQEgNGvP89ILpceEDWuNtjMzQsc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LZPWhNTp; 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="LZPWhNTp" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0206DC4AF10; Wed, 19 Nov 2025 13:42:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1763559772; bh=P/9pmsmDZmkNlTyjbeOkKXUp9EgbKAdqcxRg8cTn9uY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=LZPWhNTptIctgTzaFpTd1ekG9qrGfohBqAXAHzzaU/HV67myANBywQIS/3x/phUjm DYDIhDYRi2Ivwc7B5q7hVUqiI/vIfjCdDNCVRado+aVdcD0tmv7IO4i0D/u40bFrWT CjvOkJ2Z7lepRX0ifqfXVjCrMPqnStFWbLVY19g24sOYIuWTbQo0y1QAm4DprRGO1i aboRJ2HuKLHhg3MYwK/LN0Tzpuf27/kObDzlA9GsZ9k+CZgy9xrXEQJk0bKopSx1Dd Bv28ygxN1NRHIbPAah5Y1t2rJIGCMFf0WAQN8PfHquHssFUz8z2mldqA3t1Hi8XD3S jDXI73w0cwhrg== Date: Wed, 19 Nov 2025 15:42:45 +0200 From: Leon Romanovsky To: Christian =?iso-8859-1?Q?K=F6nig?= Cc: Bjorn Helgaas , Logan Gunthorpe , Jens Axboe , Robin Murphy , Joerg Roedel , Will Deacon , Marek Szyprowski , Jason Gunthorpe , 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: <20251119134245.GD18335@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> Precedence: bulk X-Mailing-List: linux-pci@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: <8a11b605-6ac7-48ac-8f27-22df7072e4ad@amd.com> On Wed, Nov 19, 2025 at 02:16:57PM +0100, Christian König wrote: > > > On 11/11/25 10:57, Leon Romanovsky wrote: > > From: Leon Romanovsky > > > > Add dma_buf_map() and dma_buf_unmap() helpers to convert an array of > > MMIO physical address ranges into scatter-gather tables with proper > > DMA mapping. > > > > These common functions are a starting point and support any PCI > > drivers creating mappings from their BAR's MMIO addresses. VFIO is one > > case, as shortly will be RDMA. We can review existing DRM drivers to > > refactor them separately. We hope this will evolve into routines to > > help common DRM that include mixed CPU and MMIO mappings. > > > > Compared to the dma_map_resource() abuse this implementation handles > > the complicated PCI P2P scenarios properly, especially when an IOMMU > > is enabled: > > > > - Direct bus address mapping without IOVA allocation for > > PCI_P2PDMA_MAP_BUS_ADDR, using pci_p2pdma_bus_addr_map(). This > > happens if the IOMMU is enabled but the PCIe switch ACS flags allow > > transactions to avoid the host bridge. > > > > Further, this handles the slightly obscure, case of MMIO with a > > phys_addr_t that is different from the physical BAR programming > > (bus offset). The phys_addr_t is converted to a dma_addr_t and > > accommodates this effect. This enables certain real systems to > > work, especially on ARM platforms. > > > > - Mapping through host bridge with IOVA allocation and DMA_ATTR_MMIO > > attribute for MMIO memory regions (PCI_P2PDMA_MAP_THRU_HOST_BRIDGE). > > This happens when the IOMMU is enabled and the ACS flags are forcing > > all traffic to the IOMMU - ie for virtualization systems. > > > > - Cases where P2P is not supported through the host bridge/CPU. The > > P2P subsystem is the proper place to detect this and block it. > > > > Helper functions fill_sg_entry() and calc_sg_nents() handle the > > scatter-gather table construction, splitting large regions into > > UINT_MAX-sized chunks to fit within sg->length field limits. > > > > Since the physical address based DMA API forbids use of the CPU list > > of the scatterlist this will produce a mangled scatterlist that has > > a fully zero-length and NULL'd CPU list. The list is 0 length, > > all the struct page pointers are NULL and zero sized. This is stronger > > and more robust than the existing mangle_sg_table() technique. It is > > a future project to migrate DMABUF as a subsystem away from using > > scatterlist for this data structure. > > > > Tested-by: Alex Mastro > > Tested-by: Nicolin Chen > > Signed-off-by: Leon Romanovsky > > --- > > drivers/dma-buf/dma-buf.c | 235 ++++++++++++++++++++++++++++++++++++++++++++++ > > include/linux/dma-buf.h | 18 ++++ > > 2 files changed, 253 insertions(+) > > > > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c > > index 2bcf9ceca997..cb55dff1dad5 100644 > > --- a/drivers/dma-buf/dma-buf.c > > +++ b/drivers/dma-buf/dma-buf.c > > @@ -1254,6 +1254,241 @@ void dma_buf_unmap_attachment_unlocked(struct dma_buf_attachment *attach, > > } > > EXPORT_SYMBOL_NS_GPL(dma_buf_unmap_attachment_unlocked, "DMA_BUF"); > > > > +static struct scatterlist *fill_sg_entry(struct scatterlist *sgl, size_t length, > > + dma_addr_t addr) > > +{ > > + unsigned int len, nents; > > + int i; > > + > > + nents = DIV_ROUND_UP(length, UINT_MAX); > > + for (i = 0; i < nents; i++) { > > + len = min_t(size_t, length, UINT_MAX); > > + length -= len; > > + /* > > + * DMABUF abuses scatterlist to create a scatterlist > > + * that does not have any CPU list, only the DMA list. > > + * Always set the page related values to NULL to ensure > > + * importers can't use it. The phys_addr based DMA API > > + * does not require the CPU list for mapping or unmapping. > > + */ > > + sg_set_page(sgl, NULL, 0, 0); > > + sg_dma_address(sgl) = addr + i * UINT_MAX; > > + sg_dma_len(sgl) = len; > > + sgl = sg_next(sgl); > > + } > > + > > + return sgl; > > +} > > + > > +static unsigned int calc_sg_nents(struct dma_iova_state *state, > > + struct dma_buf_phys_vec *phys_vec, > > + size_t nr_ranges, size_t size) > > +{ > > + unsigned int nents = 0; > > + size_t i; > > + > > + if (!state || !dma_use_iova(state)) { > > + for (i = 0; i < nr_ranges; i++) > > + nents += DIV_ROUND_UP(phys_vec[i].len, UINT_MAX); > > + } else { > > + /* > > + * In IOVA case, there is only one SG entry which spans > > + * for whole IOVA address space, but we need to make sure > > + * that it fits sg->length, maybe we need more. > > + */ > > + nents = DIV_ROUND_UP(size, UINT_MAX); > > + } > > + > > + return nents; > > +} > > + > > +/** > > + * struct dma_buf_dma - holds DMA mapping information > > + * @sgt: Scatter-gather table > > + * @state: DMA IOVA state relevant in IOMMU-based DMA > > + * @size: Total size of DMA transfer > > + */ > > +struct dma_buf_dma { > > + struct sg_table sgt; > > + struct dma_iova_state *state; > > + size_t size; > > +}; > > + > > +/** > > + * dma_buf_map - Returns the scatterlist table of the attachment from arrays > > + * of physical vectors. This funciton is intended for MMIO memory only. > > + * @attach: [in] attachment whose scatterlist is to be returned > > + * @provider: [in] p2pdma provider > > + * @phys_vec: [in] array of physical vectors > > + * @nr_ranges: [in] number of entries in phys_vec array > > + * @size: [in] total size of phys_vec > > + * @dir: [in] direction of DMA transfer > > + * > > + * Returns sg_table containing the scatterlist to be returned; returns ERR_PTR > > + * on error. May return -EINTR if it is interrupted by a signal. > > + * > > + * On success, the DMA addresses and lengths in the returned scatterlist are > > + * PAGE_SIZE aligned. > > + * > > + * A mapping must be unmapped by using dma_buf_unmap(). > > + */ > > +struct sg_table *dma_buf_map(struct dma_buf_attachment *attach, > > That is clearly not a good name for this function. We already have overloaded the term *mapping* with something completely different. This function performs DMA mapping, so what name do you suggest instead of dma_buf_map()? > > > + struct p2pdma_provider *provider, > > + struct dma_buf_phys_vec *phys_vec, > > + size_t nr_ranges, size_t size, > > + enum dma_data_direction dir) > > +{ > > + unsigned int nents, mapped_len = 0; > > + struct dma_buf_dma *dma; > > + struct scatterlist *sgl; > > + dma_addr_t addr; > > + size_t i; > > + int ret; > > + > > + dma_resv_assert_held(attach->dmabuf->resv); > > + > > + if (WARN_ON(!attach || !attach->dmabuf || !provider)) > > + /* This function is supposed to work on MMIO memory only */ > > + return ERR_PTR(-EINVAL); > > + > > + dma = kzalloc(sizeof(*dma), GFP_KERNEL); > > + if (!dma) > > + return ERR_PTR(-ENOMEM); > > + > > + switch (pci_p2pdma_map_type(provider, attach->dev)) { > > + case PCI_P2PDMA_MAP_BUS_ADDR: > > + /* > > + * There is no need in IOVA at all for this flow. > > + */ > > + break; > > + 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 didn't fully understand the email either. The importer needs to configure DMA and it supports only MMIO addresses. Exporter controls it by asking for peer2peer. Thanks