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Message-ID: <20260130133157.GP10992@unreal>
Date: Fri, 30 Jan 2026 15:31:57 +0200
From: Leon Romanovsky <leon@...nel.org>
To: Christian König <christian.koenig@....com>
Cc: 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>,
Thomas Hellström <thomas.hellstrom@...ux.intel.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>,
Felix Kuehling <Felix.Kuehling@....com>,
Alex Williamson <alex@...zbot.org>,
Ankit Agrawal <ankita@...dia.com>,
Vivek Kasireddy <vivek.kasireddy@...el.com>,
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 v5 4/8] vfio: Wait for dma-buf invalidation to complete
On Fri, Jan 30, 2026 at 02:21:08PM +0100, Christian König wrote:
> On 1/30/26 14:01, Leon Romanovsky wrote:
> > On Fri, Jan 30, 2026 at 09:30:59AM +0100, Christian König wrote:
> >> On 1/24/26 20:14, Leon Romanovsky wrote:
> >>> From: Leon Romanovsky <leonro@...dia.com>
> >>>
> >>> dma-buf invalidation is handled asynchronously by the hardware, so VFIO
> >>> must wait until all affected objects have been fully invalidated.
> >>>
> >>> In addition, the dma-buf exporter is expecting that all importers unmap any
> >>> buffers they previously mapped.
> >>>
> >>> Fixes: 5d74781ebc86 ("vfio/pci: Add dma-buf export support for MMIO regions")
> >>> Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> >>> ---
> >>> drivers/vfio/pci/vfio_pci_dmabuf.c | 71 ++++++++++++++++++++++++++++++++++++--
> >>> 1 file changed, 68 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> >>> index d8ceafabef48..485515629fe4 100644
> >>> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> >>> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> >>> @@ -17,6 +17,8 @@ struct vfio_pci_dma_buf {
> >>> struct dma_buf_phys_vec *phys_vec;
> >>> struct p2pdma_provider *provider;
> >>> u32 nr_ranges;
> >>> + struct kref kref;
> >>> + struct completion comp;
> >>> u8 revoked : 1;
> >>> };
> >>>
> >>> @@ -44,27 +46,46 @@ static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
> >>> return 0;
> >>> }
> >>>
> >>> +static void vfio_pci_dma_buf_done(struct kref *kref)
> >>> +{
> >>> + struct vfio_pci_dma_buf *priv =
> >>> + container_of(kref, struct vfio_pci_dma_buf, kref);
> >>> +
> >>> + complete(&priv->comp);
> >>> +}
> >>> +
> >>> static struct sg_table *
> >>> vfio_pci_dma_buf_map(struct dma_buf_attachment *attachment,
> >>> enum dma_data_direction dir)
> >>> {
> >>> struct vfio_pci_dma_buf *priv = attachment->dmabuf->priv;
> >>> + struct sg_table *ret;
> >>>
> >>> dma_resv_assert_held(priv->dmabuf->resv);
> >>>
> >>> if (priv->revoked)
> >>> return ERR_PTR(-ENODEV);
> >>>
> >>> - return dma_buf_phys_vec_to_sgt(attachment, priv->provider,
> >>> - priv->phys_vec, priv->nr_ranges,
> >>> - priv->size, dir);
> >>> + ret = dma_buf_phys_vec_to_sgt(attachment, priv->provider,
> >>> + priv->phys_vec, priv->nr_ranges,
> >>> + priv->size, dir);
> >>> + if (IS_ERR(ret))
> >>> + return ret;
> >>> +
> >>> + kref_get(&priv->kref);
> >>> + return ret;
> >>> }
> >>>
> >>> static void vfio_pci_dma_buf_unmap(struct dma_buf_attachment *attachment,
> >>> struct sg_table *sgt,
> >>> enum dma_data_direction dir)
> >>> {
> >>> + struct vfio_pci_dma_buf *priv = attachment->dmabuf->priv;
> >>> +
> >>> + dma_resv_assert_held(priv->dmabuf->resv);
> >>> +
> >>> dma_buf_free_sgt(attachment, sgt, dir);
> >>> + kref_put(&priv->kref, vfio_pci_dma_buf_done);
> >>> }
> >>>
> >>> static void vfio_pci_dma_buf_release(struct dma_buf *dmabuf)
> >>> @@ -287,6 +308,9 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
> >>> goto err_dev_put;
> >>> }
> >>>
> >>> + kref_init(&priv->kref);
> >>> + init_completion(&priv->comp);
> >>> +
> >>> /* dma_buf_put() now frees priv */
> >>> INIT_LIST_HEAD(&priv->dmabufs_elm);
> >>> down_write(&vdev->memory_lock);
> >>> @@ -326,6 +350,8 @@ void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked)
> >>> lockdep_assert_held_write(&vdev->memory_lock);
> >>>
> >>> list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) {
> >>> + unsigned long wait;
> >>> +
> >>> if (!get_file_active(&priv->dmabuf->file))
> >>> continue;
> >>>
> >>> @@ -333,7 +359,37 @@ void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked)
> >>> dma_resv_lock(priv->dmabuf->resv, NULL);
> >>> priv->revoked = revoked;
> >>> dma_buf_invalidate_mappings(priv->dmabuf);
> >>> + dma_resv_wait_timeout(priv->dmabuf->resv,
> >>> + DMA_RESV_USAGE_BOOKKEEP, false,
> >>> + MAX_SCHEDULE_TIMEOUT);
> >>> dma_resv_unlock(priv->dmabuf->resv);
> >>> + if (revoked) {
> >>> + kref_put(&priv->kref, vfio_pci_dma_buf_done);
> >>> + /* Let's wait till all DMA unmap are completed. */
> >>> + wait = wait_for_completion_timeout(
> >>> + &priv->comp, secs_to_jiffies(1));
> >>> + /*
> >>> + * If you see this WARN_ON, it means that
> >>> + * importer didn't call unmap in response to
> >>> + * dma_buf_invalidate_mappings() which is not
> >>> + * allowed.
> >>> + */
> >>> + WARN(!wait,
> >>> + "Timed out waiting for DMABUF unmap, importer has a broken invalidate_mapping()");
> >>
> >> You can do the revoke to do your resource management, for example re-use the backing store for something else.
> >>
> >> But it is mandatory that you keep the mapping around indefinitely until the importer closes it.
> >>
> >> Before that you can't do things like runtime PM or remove or anything which would make the DMA addresses invalid.
> >>
> >> As far as I can see vfio_pci_dma_buf_move() is used exactly for that use case so this here is an absolutely clear NAK from my side for this approach.
> >>
> >> You can either split up the functionality of vfio_pci_dma_buf_move() into vfio_pci_dma_buf_invalidate_mappings() and vfio_pci_dma_buf_flush() and then call the later whenever necessary or you keep it in one function and block everybody until the importer has dropped all mappings.
> >
> > No problem, I can change it to be:
> >
> > diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> > index d087d018d547..53772a84c93b 100644
> > --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> > +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> > @@ -357,23 +357,7 @@ void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked)
> > dma_resv_unlock(priv->dmabuf->resv);
> > if (revoked) {
> > kref_put(&priv->kref, vfio_pci_dma_buf_done);
> > - /*
> > - * Let's wait for 1 second till all DMA unmap
> > - * are completed. It is supposed to catch dma-buf
> > - * importers which lied about their support
> > - * of dmabuf revoke. See dma_buf_invalidate_mappings()
> > - * for the expected behaviour.
> > - */
> > - wait = wait_for_completion_timeout(
> > - &priv->comp, secs_to_jiffies(1));
> > - /*
> > - * If you see this WARN_ON, it means that
> > - * importer didn't call unmap in response to
> > - * dma_buf_invalidate_mappings() which is not
> > - * allowed.
> > - */
> > - WARN(!wait,
> > - "Timed out waiting for DMABUF unmap, importer has a broken invalidate_mapping()");
> > + wait_for_completion(&priv->comp);
> > } else {
> > /*
> > * Kref is initialize again, because when revoke
> >
> > Do you want me to send v6?
>
> That would work for me.
>
> Question is if you really want to do it this way? See usually exporters try to avoid blocking such functions.
>
> What exporters usually do instead is to grab references, e.g. call pm_runtime_get_sync() when either a DMA-buf, a DMA-buf attachment or in your case here a mapping of this attachment is made.
I view this as an enhancement that can be addressed later down the road.
>
> But all of this is just a suggestion, if you are fine with blocking then feel free to add my rb.
It is fine for initial version. We need to start somewhere.
Thanks
>
> Regards,
> Christian.
>
> >
> > Thanks
> >
> >>
> >>> + } else {
> >>> + /*
> >>> + * Kref is initialize again, because when revoke
> >>> + * was performed the reference counter was decreased
> >>> + * to zero to trigger completion.
> >>> + */
> >>> + kref_init(&priv->kref);
> >>> + /*
> >>> + * There is no need to wait as no mapping was
> >>> + * performed when the previous status was
> >>> + * priv->revoked == true.
> >>> + */
> >>> + reinit_completion(&priv->comp);
> >>> + }
> >>> }
> >>> fput(priv->dmabuf->file);
> >>
> >> This is also extremely questionable. Why doesn't the dmabuf have a reference while on the linked list?
> >>
> >> Regards,
> >> Christian.
> >>
> >>> }
> >>> @@ -346,6 +402,8 @@ void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev)
> >>>
> >>> down_write(&vdev->memory_lock);
> >>> list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) {
> >>> + unsigned long wait;
> >>> +
> >>> if (!get_file_active(&priv->dmabuf->file))
> >>> continue;
> >>>
> >>> @@ -354,7 +412,14 @@ void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev)
> >>> priv->vdev = NULL;
> >>> priv->revoked = true;
> >>> dma_buf_invalidate_mappings(priv->dmabuf);
> >>> + dma_resv_wait_timeout(priv->dmabuf->resv,
> >>> + DMA_RESV_USAGE_BOOKKEEP, false,
> >>> + MAX_SCHEDULE_TIMEOUT);
> >>> dma_resv_unlock(priv->dmabuf->resv);
> >>> + kref_put(&priv->kref, vfio_pci_dma_buf_done);
> >>> + wait = wait_for_completion_timeout(&priv->comp,
> >>> + secs_to_jiffies(1));
> >>> + WARN_ON(!wait);
> >>> vfio_device_put_registration(&vdev->vdev);
> >>> fput(priv->dmabuf->file);
> >>> }
> >>>
> >>
> >>
>
>
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