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

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Message-ID: <20250930143408.GI2942991@nvidia.com>
Date: Tue, 30 Sep 2025 11:34:08 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Shameer Kolothum <skolothumtho@...dia.com>
Cc: Leon Romanovsky <leon@...nel.org>,
	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" <dri-devel@...ts.freedesktop.org>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	Jens Axboe <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-pci@...r.kernel.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 10/10] vfio/pci: Add dma-buf export support for MMIO
 regions

On Tue, Sep 30, 2025 at 12:50:47PM +0000, Shameer Kolothum wrote:

> This is where hisi_acc reports a different BAR size as it tries to hide
> the migration control region from Guest access.

I think for now we should disable DMABUF for any PCI driver that
implements a VFIO_DEVICE_GET_REGION_INFO

For a while I've wanted to further reduce the use of the ioctl
multiplexer, so maybe this series:

https://github.com/jgunthorpe/linux/commits/vfio_get_region_info_op/

And then the dmabuf code can check if the ops are set to the generic
or not and disable itself automatically.

Otherwise perhaps route the dmabuf through an op and deliberately omit
it (with a comment!) from hisi, virtio, nvgrace.

We need to route it through an op anyhow as those three drivers will
probably eventually want to implement their own version.

Jason