linux-kernel - Re: [PATCH v2 4/4] vfio: Add pinned interface to perform revoke semantics

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Message-ID: <a397ff1e-615f-4873-98a9-940f9c16f85c@amd.com>
Date: Mon, 19 Jan 2026 15:21:11 +0100
From: Christian König <christian.koenig@....com>
To: Leon Romanovsky <leon@...nel.org>
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>,
 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 4/4] vfio: Add pinned interface to perform revoke
 semantics

On 1/19/26 14:02, Leon Romanovsky wrote:
> On Mon, Jan 19, 2026 at 01:12:45PM +0100, Christian König wrote:
>> On 1/18/26 13:08, Leon Romanovsky wrote:
>>> From: Leon Romanovsky <leonro@...dia.com>
>>>
>>> DMABUF ->pin() interface is called when the DMABUF importer perform
>>> its DMA mapping, so let's use this opportunity to check if DMABUF
>>> exporter revoked its buffer or not.
>>>
>>> Signed-off-by: Leon Romanovsky <leonro@...dia.com>
>>> ---
>>>  drivers/vfio/pci/vfio_pci_dmabuf.c | 16 ++++++++++++++++
>>>  1 file changed, 16 insertions(+)
>>>
>>> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
>>> index d4d0f7d08c53..af9c315ddf71 100644
>>> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
>>> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
>>> @@ -20,6 +20,20 @@ struct vfio_pci_dma_buf {
>>>  	u8 revoked : 1;
>>>  };
>>>  
>>> +static int vfio_pci_dma_buf_pin(struct dma_buf_attachment *attachment)
>>> +{
>>> +	struct vfio_pci_dma_buf *priv = attachment->dmabuf->priv;
>>> +
>>> +	dma_resv_assert_held(priv->dmabuf->resv);
>>> +
>>> +	return dma_buf_attachment_is_revoke(attachment) ? 0 : -EOPNOTSUPP;
>>
>> It's probably better to do that check in vfio_pci_dma_buf_attach.
> 
> I assume you are proposing to add this check in both
> vfio_pci_dma_buf_attach() and vfio_pci_dma_buf_pin(). Otherwise,
> importers that lack .invalidate_mapping() will invoke dma_buf_pin()
> and will not fail.

vfio_pci_dma_buf_attach() alone should be sufficient. It is always called, even for importers lacking invalidate_mapping().

Regards,
Christian.

> 
>>
>> And BTW the function vfio_pci_dma_buf_move() seems to be broken:
>>
>> void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked)
>> {
>>         struct vfio_pci_dma_buf *priv;
>>         struct vfio_pci_dma_buf *tmp;
>>
>>         lockdep_assert_held_write(&vdev->memory_lock);
>>
>>         list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) {
>>                 if (!get_file_active(&priv->dmabuf->file))
>>                         continue;
>>
>>                 if (priv->revoked != revoked) {
>>                         dma_resv_lock(priv->dmabuf->resv, NULL);
>>                         priv->revoked = revoked;
>>                         dma_buf_move_notify(priv->dmabuf);
>>
>> A dma_buf_move_notify() just triggers asynchronous invalidation of the mapping!
>>
>> You need to use dma_resv_wait() to wait for that to finish.
> 
> We (VFIO and IOMMUFD) followed the same pattern used in  
> amdgpu_bo_move_notify(), which also does not wait.
> 
> I'll add wait here.
> 
> Thanks
> 
>>
>>                         dma_resv_unlock(priv->dmabuf->resv);
>>                 }
>>                 fput(priv->dmabuf->file);
>>         }
>> }
>>
>> Regards,
>> Christian.
>>
>>
>>> +}
>>> +
>>> +static void vfio_pci_dma_buf_unpin(struct dma_buf_attachment *attachment)
>>> +{
>>> +	/* Do nothing */
>>> +}
>>> +
>>>  static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
>>>  				   struct dma_buf_attachment *attachment)
>>>  {
>>> @@ -76,6 +90,8 @@ static void vfio_pci_dma_buf_release(struct dma_buf *dmabuf)
>>>  }
>>>  
>>>  static const struct dma_buf_ops vfio_pci_dmabuf_ops = {
>>> +	.pin = vfio_pci_dma_buf_pin,
>>> +	.unpin = vfio_pci_dma_buf_unpin,
>>>  	.attach = vfio_pci_dma_buf_attach,
>>>  	.map_dma_buf = vfio_pci_dma_buf_map,
>>>  	.unmap_dma_buf = vfio_pci_dma_buf_unmap,
>>>
>>