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Message-ID: <20260130130131.GO10992@unreal>
Date: Fri, 30 Jan 2026 15:01:31 +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 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?
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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