linux-kernel - Re: [PATCH v4 08/10] vfio/pci: Enable peer-to-peer DMA transactions by default

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Message-ID: <20250929151745.439be1ec.alex.williamson@redhat.com>
Date: Mon, 29 Sep 2025 15:17:45 -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>, 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 v4 08/10] vfio/pci: Enable peer-to-peer DMA transactions
 by default

On Sun, 28 Sep 2025 17:50:18 +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 | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index 7dcf5439dedc..608af135308e 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -28,6 +28,9 @@
>  #include <linux/nospec.h>
>  #include <linux/sched/mm.h>
>  #include <linux/iommufd.h>
> +#ifdef CONFIG_VFIO_PCI_DMABUF
> +#include <linux/pci-p2pdma.h>
> +#endif
>  #if IS_ENABLED(CONFIG_EEH)
>  #include <asm/eeh.h>
>  #endif
> @@ -2085,6 +2088,7 @@ int vfio_pci_core_init_dev(struct vfio_device *core_vdev)
>  {
>  	struct vfio_pci_core_device *vdev =
>  		container_of(core_vdev, struct vfio_pci_core_device, vdev);
> +	int __maybe_unused ret;
>  
>  	vdev->pdev = to_pci_dev(core_vdev->dev);
>  	vdev->irq_type = VFIO_PCI_NUM_IRQS;
> @@ -2094,6 +2098,11 @@ 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);
> +#ifdef CONFIG_VFIO_PCI_DMABUF
> +	ret = pcim_p2pdma_init(vdev->pdev);
> +	if (ret)
> +		return ret;
> +#endif
>  	init_rwsem(&vdev->memory_lock);
>  	xa_init(&vdev->ctx);
>  

What breaks if we don't test the return value and remove all the
#ifdefs?  The feature call should fail if we don't have a provider but
that seems more robust than failing to register the device.  Thanks,

Alex