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

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Message-ID: <20250806160201.2b72e7a0.alex.williamson@redhat.com>
Date: Wed, 6 Aug 2025 16:02:01 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Leon Romanovsky <leon@...nel.org>
Cc: Leon Romanovsky <leonro@...dia.com>, Jason Gunthorpe <jgg@...dia.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Bjorn Helgaas
 <bhelgaas@...gle.com>, Christian König
 <christian.koenig@....com>, Christoph Hellwig <hch@....de>,
 dri-devel@...ts.freedesktop.org, iommu@...ts.linux.dev, Jens Axboe
 <axboe@...nel.dk>, 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, Logan Gunthorpe
 <logang@...tatee.com>, 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 v1 08/10] vfio/pci: Enable peer-to-peer DMA transactions
 by default

On Mon,  4 Aug 2025 16:00:43 +0300
Leon Romanovsky <leon@...nel.org> wrote:

> From: Leon Romanovsky <leonro@...dia.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@...dia.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,

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 */