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

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Message-ID: <20250930180434.GK2942991@nvidia.com>
Date: Tue, 30 Sep 2025 15:04:34 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Alex Williamson <alex.williamson@...hat.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, Sep 30, 2025 at 10:52:47AM -0600, Alex Williamson wrote:
> 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? 

Could work to start! That's a good idea, then we don't have
dependencies.

> Your proposal would have better granularity, but

Yes, that was my thinking

> 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.

Small steps..

I added some more commits that remove the userspace buffer and all the
duplicated code too.

Jason