Message-ID: <20260119105312.GJ13201@unreal>
Date: Mon, 19 Jan 2026 12:53:12 +0200
From: Leon Romanovsky <leon@...nel.org>
To: Christian König <christian.koenig@....com>
Cc: Thomas Hellström <thomas.hellstrom@...ux.intel.com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	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 11:20:46AM +0100, Christian König wrote:
> On 1/19/26 10:27, 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.  
> 
> Please don't document that. This is specific exporter behavior and doesn't belong into DMA-buf at all.
> 
> >> 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.
> 
> I have to clearly reject that.
> 
> It's the job of the exporter to reject such calls with an appropriate error and not the importer to not make them.

Current code behaves as expected: the exporter rejects mapping attempts after
.invalidate_mapping is called, and handles the logic internally.

However, it is not clear what exactly you are proposing. In v1 — which you
objected to — I suggested negotiating revoke support along with the logic for
rejecting mappings in the dma-buf core. In this version, you object to placing
the rejection logic in the 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
> 
> Yes, that case is well known.
> 
> > this sounds like it's not just undocumented but also in some cases
> > unimplemented. The xe driver for one doesn't expect move_notify() to be
> > called on pinned buffers,
> 
> And that is what we need to change. See move_notify can happen on pinned buffers currently as well.
> 
> For example in the case of PCI hot unplug. After pinning we just don't call it for memory management needs any more.
> 
> We just haven't documented that properly.
> 
> > 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?
> 
> That's what this patch set here should do, yes.
> 
> Regards,
> Christian.
> 
> > 
> > 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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