linux-kernel - Re: [PATCH 05/10] PCI/P2PDMA: Export pci_p2pdma_map

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Message-ID: <20250728164136.GD402218@unreal>
Date: Mon, 28 Jul 2025 19:41:36 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Logan Gunthorpe <logang@...tatee.com>
Cc: Jason Gunthorpe <jgg@...dia.com>, Christoph Hellwig <hch@....de>,
	Alex Williamson <alex.williamson@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Christian König <christian.koenig@....com>,
	dri-devel@...ts.freedesktop.org, iommu@...ts.linux.dev,
	Jens Axboe <axboe@...nel.dk>,
	Jérôme Glisse <jglisse@...hat.com>,
	Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
	linaro-mm-sig@...ts.linaro.org, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	linux-mm@...ck.org, linux-pci@...r.kernel.org,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Vivek Kasireddy <vivek.kasireddy@...el.com>,
	Will Deacon <will@...nel.org>
Subject: Re: [PATCH 05/10] PCI/P2PDMA: Export pci_p2pdma_map_type() function

On Mon, Jul 28, 2025 at 10:12:31AM -0600, Logan Gunthorpe wrote:
> 
> 
> On 2025-07-27 13:05, Jason Gunthorpe wrote:
> > On Fri, Jul 25, 2025 at 10:30:46AM -0600, Logan Gunthorpe wrote:
> >>
> >>
> >> On 2025-07-24 02:13, Leon Romanovsky wrote:
> >>> On Thu, Jul 24, 2025 at 10:03:13AM +0200, Christoph Hellwig wrote:
> >>>> On Wed, Jul 23, 2025 at 04:00:06PM +0300, Leon Romanovsky wrote:
> >>>>> From: Leon Romanovsky <leonro@...dia.com>
> >>>>>
> >>>>> Export the pci_p2pdma_map_type() function to allow external modules
> >>>>> and subsystems to determine the appropriate mapping type for P2PDMA
> >>>>> transfers between a provider and target device.
> >>>>
> >>>> External modules have no business doing this.
> >>>
> >>> VFIO PCI code is built as module. There is no way to access PCI p2p code
> >>> without exporting functions in it.
> >>
> >> The solution that would make more sense to me would be for either
> >> dma_iova_try_alloc() or another helper in dma-iommu.c to handle the
> >> P2PDMA case.
> > 
> > This has nothing to do with dma-iommu.c, the decisions here still need
> > to be made even if dma-iommu.c is not compiled in.
> 
> Doesn't it though? Every single call in patch 10 to the newly exported
> PCI functions calls into the the dma-iommu functions. If there were
> non-iommu paths then I would expect the code would use the regular DMA
> api directly which would then call in to dma-iommu.

If p2p type is PCI_P2PDMA_MAP_BUS_ADDR, there will no dma-iommu and DMA
at all.

+static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
+				   struct dma_buf_attachment *attachment)
+{
+	struct vfio_pci_dma_buf *priv = dmabuf->priv;
+
+	if (!attachment->peer2peer)
+		return -EOPNOTSUPP;
+
+	if (priv->revoked)
+		return -ENODEV;
+
+	switch (pci_p2pdma_map_type(priv->vdev->provider, attachment->dev)) {
+	case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
+		break;
+	case PCI_P2PDMA_MAP_BUS_ADDR:
+		/*
+		 * There is no need in IOVA at all for this flow.
+		 * We rely on attachment->priv == NULL as a marker
+		 * for this mode.
+		 */
+		return 0;
+	default:
+		return -EINVAL;
+	}
+
+	attachment->priv = kzalloc(sizeof(struct dma_iova_state), GFP_KERNEL);
+	if (!attachment->priv)
+		return -ENOMEM;
+
+	dma_iova_try_alloc(attachment->dev, attachment->priv, 0, priv->phys_vec.len);
+	return 0;
+}