Re: [PATCH v1 08/10] vfio/pci: Enable peer-to-peer DMA transactions by default

Leon Romanovsky
2025-09-02T12:58:29Z

On Wed, Aug 06, 2025 at 04:02:01PM -0600, Alex Williamson wrote:
> On Mon, 4 Aug 2025 16:00:43 +0300
> Leon Romanovsky <leon@kernel.org> wrote:
>
> > From: Leon Romanovsky <leonro@nvidia.com>
> >
> > Make sure that all VFIO PCI devices have peer-to-peer capabilities
> > enables, so we would be able to export their MMIO memory through DMABUF,
> >
> > Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> > ---
> > drivers/vfio/pci/vfio_pci_core.c | 4 ++++
> > include/linux/vfio_pci_core.h | 1 +
> > 2 files changed, 5 insertions(+)
> >
> > diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> > index 31bdb9110cc0f..df9a32d3deac9 100644
> > --- a/drivers/vfio/pci/vfio_pci_core.c
> > +++ b/drivers/vfio/pci/vfio_pci_core.c
> > @@ -28,6 +28,7 @@
> > #include <linux/nospec.h>
> > #include <linux/sched/mm.h>
> > #include <linux/iommufd.h>
> > +#include <linux/pci-p2pdma.h>
> > #if IS_ENABLED(CONFIG_EEH)
> > #include <asm/eeh.h>
> > #endif
> > @@ -2088,6 +2089,9 @@ int vfio_pci_core_init_dev(struct vfio_device *core_vdev)
> > INIT_LIST_HEAD(&vdev->dummy_resources_list);
> > INIT_LIST_HEAD(&vdev->ioeventfds_list);
> > INIT_LIST_HEAD(&vdev->sriov_pfs_item);
> > + vdev->provider = pci_p2pdma_enable(vdev->pdev);
> > + if (IS_ERR(vdev->provider))
> > + return PTR_ERR(vdev->provider);
>
> I think this just made all vfio-pci drivers functionally dependent on
> CONFIG_PCI_P2PDMA. Seems at best exporting a dma-buf should be
> restricted if this fails. Thanks,

It is temporary solution in next patch "vfio/pci: Add dma-buf export
support for MMIO regions", the strict ifdef is added.

2107 #ifdef CONFIG_VFIO_PCI_DMABUF
2108 vdev->provider = pci_p2pdma_enable(vdev->pdev);
2109 if (IS_ERR(vdev->provider))
2110 return PTR_ERR(vdev->provider);
2111
2112 INIT_LIST_HEAD(&vdev->dmabufs);
2113 #endif

I will split "vfio/pci: Add dma-buf export ..." patch to introduce CONFIG_VFIO_PCI_DMABUF
before this "vfio/pci: Enable peer-to-peer ..." patch.

Thanks

>
> Alex
>
> > init_rwsem(&vdev->memory_lock);
> > xa_init(&vdev->ctx);
> >
> > diff --git a/include/linux/vfio_pci_core.h b/include/linux/vfio_pci_core.h
> > index fbb472dd99b36..b017fae251811 100644
> > --- a/include/linux/vfio_pci_core.h
> > +++ b/include/linux/vfio_pci_core.h
> > @@ -94,6 +94,7 @@ struct vfio_pci_core_device {
> > struct vfio_pci_core_device *sriov_pf_core_dev;
> > struct notifier_block nb;
> > struct rw_semaphore memory_lock;
> > + struct p2pdma_provider *provider;
> > };
> >
> > /* Will be exported for vfio pci drivers usage */
>