Message-ID: <20260119102000.GH13201@unreal>
Date: Mon, 19 Jan 2026 12:20:00 +0200
From: Leon Romanovsky <leon@...nel.org>
To: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
Cc: Sumit Semwal <sumit.semwal@...aro.org>,
	Christian König <christian.koenig@....com>,
	Alex Deucher <alexander.deucher@....com>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Gerd Hoffmann <kraxel@...hat.com>,
	Dmitry Osipenko <dmitry.osipenko@...labora.com>,
	Gurchetan Singh <gurchetansingh@...omium.org>,
	Chia-I Wu <olvaffe@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Jason Gunthorpe <jgg@...pe.ca>, Kevin Tian <kevin.tian@...el.com>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	Alex Williamson <alex@...zbot.org>, linux-media@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
	linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
	virtualization@...ts.linux.dev, intel-xe@...ts.freedesktop.org,
	linux-rdma@...r.kernel.org, iommu@...ts.linux.dev,
	kvm@...r.kernel.org
Subject: Re: [PATCH v2 0/4] dma-buf: document revoke mechanism to invalidate
 shared buffers

On Mon, Jan 19, 2026 at 10:27:00AM +0100, Thomas Hellström wrote:
> On Mon, 2026-01-19 at 09:52 +0200, Leon Romanovsky wrote:
> > On Sun, Jan 18, 2026 at 03:16:25PM +0100, Thomas Hellström wrote:
> > > Hi, Leon,
> > > 
> > > On Sun, 2026-01-18 at 14:08 +0200, Leon Romanovsky wrote:
> > > > Changelog:
> > > > v2:
> > > >  * Changed series to document the revoke semantics instead of
> > > >    implementing it.
> > > > v1:
> > > > https://patch.msgid.link/20260111-dmabuf-revoke-v1-0-fb4bcc8c259b@nvidia.com
> > > > 
> > > > -----------------------------------------------------------------
> > > > ----
> > > > ----
> > > > This series documents a dma-buf “revoke” mechanism: to allow a
> > > > dma-
> > > > buf
> > > > exporter to explicitly invalidate (“kill”) a shared buffer after
> > > > it
> > > > has
> > > > been distributed to importers, so that further CPU and device
> > > > access
> > > > is
> > > > prevented and importers reliably observe failure.
> > > > 
> > > > The change in this series is to properly document and use
> > > > existing
> > > > core
> > > > “revoked” state on the dma-buf object and a corresponding
> > > > exporter-
> > > > triggered
> > > > revoke operation. Once a dma-buf is revoked, new access paths are
> > > > blocked so
> > > > that attempts to DMA-map, vmap, or mmap the buffer fail in a
> > > > consistent way.
> > > 
> > > This sounds like it does not match how many GPU-drivers use the
> > > move_notify() callback.
> > 
> > No change for them.
> > 
> > > 
> > > move_notify() would typically invalidate any device maps and any
> > > asynchronous part of that invalidation would be complete when the
> > > dma-
> > > buf's reservation object becomes idle WRT DMA_RESV_USAGE_BOOKKEEP
> > > fences.
> > 
> > This part has not changed and remains the same for the revocation
> > flow as well.
> > 
> > > 
> > > However, the importer could, after obtaining the resv lock, obtain
> > > a
> > > new map using dma_buf_map_attachment(), and I'd assume the CPU maps
> > > work in the same way, I.E. move_notify() does not *permanently*
> > > revoke
> > > importer access.
> > 
> > This part diverges by design and is documented to match revoke
> > semantics.  
> > It defines what must occur after the exporter requests that the
> > buffer be  
> > "killed". An importer that follows revoke semantics will not attempt
> > to call  
> > dma_buf_map_attachment(), and the exporter will block any remapping
> > attempts  
> > regardless. See the priv->revoked flag in the VFIO exporter.
> > 
> > In addition, in this email thread, Christian explains that revoke
> > semantics already exists, with the combination of dma_buf_pin and
> > dma_buf_move_notify, just not documented:
> > https://lore.kernel.org/all/f7f1856a-44fa-44af-b496-eb1267a05d11@amd.com/
> 
> 
> Hmm,
> 
> Considering 
> 
> https://elixir.bootlin.com/linux/v6.19-rc5/source/drivers/infiniband/core/umem_dmabuf.c#L192
> 
> this sounds like it's not just undocumented but also in some cases
> unimplemented.

Yes, it was discussed later in the thread https://lore.kernel.org/all/20260112153503.GF745888@ziepe.ca/.
RDMA will need some adjustments later, but first we need to document the
existing semantics.

> The xe driver for one doesn't expect move_notify() to be
> called on pinned buffers, so if that is indeed going to be part of the
> dma-buf protocol,  wouldn't support for that need to be advertised by
> the importer?

This is what Jason proposed with "enum dma_buf_move_notify_level", but
for some reason we got no responses.

Thanks

> 
> Thanks,
> Thomas
> 
> > 
> > Thanks
> > 
> > > 
> > > /Thomas
> > > 
> > > 
> > > > 
> > > > Thanks
> > > > 
> > > > Cc: linux-media@...r.kernel.org
> > > > Cc: dri-devel@...ts.freedesktop.org
> > > > Cc: linaro-mm-sig@...ts.linaro.org
> > > > Cc: linux-kernel@...r.kernel.org
> > > > Cc: amd-gfx@...ts.freedesktop.org
> > > > Cc: virtualization@...ts.linux.dev
> > > > Cc: intel-xe@...ts.freedesktop.org
> > > > Cc: linux-rdma@...r.kernel.org
> > > > Cc: iommu@...ts.linux.dev
> > > > Cc: kvm@...r.kernel.org
> > > > To: Sumit Semwal <sumit.semwal@...aro.org>
> > > > To: Christian König <christian.koenig@....com>
> > > > To: Alex Deucher <alexander.deucher@....com>
> > > > To: David Airlie <airlied@...il.com>
> > > > To: Simona Vetter <simona@...ll.ch>
> > > > To: Gerd Hoffmann <kraxel@...hat.com>
> > > > To: Dmitry Osipenko <dmitry.osipenko@...labora.com>
> > > > To: Gurchetan Singh <gurchetansingh@...omium.org>
> > > > To: Chia-I Wu <olvaffe@...il.com>
> > > > To: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
> > > > To: Maxime Ripard <mripard@...nel.org>
> > > > To: Thomas Zimmermann <tzimmermann@...e.de>
> > > > To: Lucas De Marchi <lucas.demarchi@...el.com>
> > > > To: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
> > > > To: Rodrigo Vivi <rodrigo.vivi@...el.com>
> > > > To: Jason Gunthorpe <jgg@...pe.ca>
> > > > To: Leon Romanovsky <leon@...nel.org>
> > > > To: Kevin Tian <kevin.tian@...el.com>
> > > > To: Joerg Roedel <joro@...tes.org>
> > > > To: Will Deacon <will@...nel.org>
> > > > To: Robin Murphy <robin.murphy@....com>
> > > > To: Alex Williamson <alex@...zbot.org>
> > > > 
> > > > ---
> > > > Leon Romanovsky (4):
> > > >       dma-buf: Rename .move_notify() callback to a clearer
> > > > identifier
> > > >       dma-buf: Document revoke semantics
> > > >       iommufd: Require DMABUF revoke semantics
> > > >       vfio: Add pinned interface to perform revoke semantics
> > > > 
> > > >  drivers/dma-buf/dma-buf.c                   |  6 +++---
> > > >  drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c |  4 ++--
> > > >  drivers/gpu/drm/virtio/virtgpu_prime.c      |  2 +-
> > > >  drivers/gpu/drm/xe/tests/xe_dma_buf.c       |  6 +++---
> > > >  drivers/gpu/drm/xe/xe_dma_buf.c             |  2 +-
> > > >  drivers/infiniband/core/umem_dmabuf.c       |  4 ++--
> > > >  drivers/infiniband/hw/mlx5/mr.c             |  2 +-
> > > >  drivers/iommu/iommufd/pages.c               | 11 +++++++++--
> > > >  drivers/vfio/pci/vfio_pci_dmabuf.c          | 16
> > > > ++++++++++++++++
> > > >  include/linux/dma-buf.h                     | 25
> > > > ++++++++++++++++++++++---
> > > >  10 files changed, 60 insertions(+), 18 deletions(-)
> > > > ---
> > > > base-commit: 9ace4753a5202b02191d54e9fdf7f9e3d02b85eb
> > > > change-id: 20251221-dmabuf-revoke-b90ef16e4236
> > > > 
> > > > Best regards,
> > > > --  
> > > > Leon Romanovsky <leonro@...dia.com>
> > > > 
> > > 
> 

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