linux-kernel - Re: [PATCH v4 07/10] vfio/pci: Add dma-buf export config for MMIO regions

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Message-ID: <20250929151740.21f001e3.alex.williamson@redhat.com>
Date: Mon, 29 Sep 2025 15:17:40 -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 07/10] vfio/pci: Add dma-buf export config for MMIO
 regions

On Sun, 28 Sep 2025 17:50:17 +0300
Leon Romanovsky <leon@...nel.org> wrote:

> From: Leon Romanovsky <leonro@...dia.com>
> 
> Add new kernel config which indicates support for dma-buf export
> of MMIO regions, which implementation is provided in next patches.
> 
> Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> ---
>  drivers/vfio/pci/Kconfig | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/drivers/vfio/pci/Kconfig b/drivers/vfio/pci/Kconfig
> index 2b0172f54665..55ae888bf26a 100644
> --- a/drivers/vfio/pci/Kconfig
> +++ b/drivers/vfio/pci/Kconfig
> @@ -55,6 +55,26 @@ config VFIO_PCI_ZDEV_KVM
>  
>  	  To enable s390x KVM vfio-pci extensions, say Y.
>  
> +config VFIO_PCI_DMABUF
> +	bool "VFIO PCI extensions for DMA-BUF"
> +	depends on VFIO_PCI_CORE
> +	depends on PCI_P2PDMA && DMA_SHARED_BUFFER
> +	default y
> +	help
> +	  Enable support for VFIO PCI extensions that allow exporting
> +	  device MMIO regions as DMA-BUFs for peer devices to access via
> +	  peer-to-peer (P2P) DMA.
> +
> +	  This feature enables a VFIO-managed PCI device to export a portion
> +	  of its MMIO BAR as a DMA-BUF file descriptor, which can be passed
> +	  to other userspace drivers or kernel subsystems capable of
> +	  initiating DMA to that region.
> +
> +	  Say Y here if you want to enable VFIO DMABUF-based MMIO export
> +	  support for peer-to-peer DMA use cases.
> +
> +	  If unsure, say N.
> +
>  source "drivers/vfio/pci/mlx5/Kconfig"
>  
>  source "drivers/vfio/pci/hisilicon/Kconfig"

This is only necessary if we think there's a need to build a kernel with
P2PDMA and VFIO_PCI, but not VFIO_PCI_DMABUF.  Does that need really
exist?

I also find it unusual to create the Kconfig before adding the
supporting code.  Maybe this could be popped to the end or rolled into
the last patch if we decided to keep it.  Thanks,

Alex