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

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Message-ID: <20250930105247.1935b553.alex.williamson@redhat.com>
Date: Tue, 30 Sep 2025 10:52:47 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Shameer Kolothum <skolothumtho@...dia.com>, Leon Romanovsky
 <leon@...nel.org>, 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, 30 Sep 2025 11:34:08 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> 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.

Can't we basically achieve the same by testing the ioctl is
vfio_pci_core_ioctl?  Your proposal would have better granularity, but
we'd probably want an ops callback that we can use without a userspace
buffer to get the advertised region size if we ever want to support a
device that both modifies the size of the region relative to the BAR
and supports p2p.  Thanks,

Alex