linux-kernel - Re: [PATCH RFC 1/4] vdpa: introduce .reset

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Date:   Tue, 15 Aug 2023 12:49:15 -0700
From:   Si-Wei Liu <si-wei.liu@...cle.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     eperezma@...hat.com, gal@...dia.com, linux-kernel@...r.kernel.org,
        mst@...hat.com, virtualization@...ts.linux-foundation.org,
        xuanzhuo@...ux.alibaba.com
Subject: Re: [PATCH RFC 1/4] vdpa: introduce .reset_map operation callback

On 8/14/2023 7:21 PM, Jason Wang wrote:
> On Tue, Aug 15, 2023 at 9:46 AM Si-Wei Liu <si-wei.liu@...cle.com> wrote:
>> Signed-off-by: Si-Wei Liu <si-wei.liu@...cle.com>
>> ---
>>   include/linux/vdpa.h | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/include/linux/vdpa.h b/include/linux/vdpa.h
>> index db1b0ea..3a3878d 100644
>> --- a/include/linux/vdpa.h
>> +++ b/include/linux/vdpa.h
>> @@ -314,6 +314,12 @@ struct vdpa_map_file {
>>    *                             @iova: iova to be unmapped
>>    *                             @size: size of the area
>>    *                             Returns integer: success (0) or error (< 0)
>> + * @reset_map:                 Reset device memory mapping (optional)
>> + *                             Needed for device that using device
>> + *                             specific DMA translation (on-chip IOMMU)
> This exposes the device internal to the upper layer which is not optimal.
Not sure what does it mean by "device internal", but this op callback 
just follows existing convention to describe what vdpa parent this API 
targets.

  * @set_map:                    Set device memory mapping (optional)
  *                              Needed for device that using device
  *                              specific DMA translation (on-chip IOMMU)
:
:
  * @dma_map:                    Map an area of PA to IOVA (optional)
  *                              Needed for device that using device
  *                              specific DMA translation (on-chip IOMMU)
  *                              and preferring incremental map.
:
:
  * @dma_unmap:                  Unmap an area of IOVA (optional but
  *                              must be implemented with dma_map)
  *                              Needed for device that using device
  *                              specific DMA translation (on-chip IOMMU)
  *                              and preferring incremental unmap.

> Btw, what's the difference between this and a simple
>
> set_map(NULL)?
I don't think parent drivers support this today - they can accept 
non-NULL iotlb containing empty map entry, but not a NULL iotlb. The 
behavior is undefined or it even causes panic when a NULL iotlb is 
passed in. Further this doesn't work with .dma_map parent drivers.

The reason why a new op is needed or better is because it allows 
userspace to tell apart different reset behavior from the older kernel 
(via the F_IOTLB_PERSIST feature bit in patch 4), while this behavior 
could vary between parent drivers.

Regards,
-Siwei

>
> Thanks
>
>> + *                             @vdev: vdpa device
>> + *                             @asid: address space identifier
>> + *                             Returns integer: success (0) or error (< 0)
>>    * @get_vq_dma_dev:            Get the dma device for a specific
>>    *                             virtqueue (optional)
>>    *                             @vdev: vdpa device
>> @@ -390,6 +396,7 @@ struct vdpa_config_ops {
>>                         u64 iova, u64 size, u64 pa, u32 perm, void *opaque);
>>          int (*dma_unmap)(struct vdpa_device *vdev, unsigned int asid,
>>                           u64 iova, u64 size);
>> +       int (*reset_map)(struct vdpa_device *vdev, unsigned int asid);
>>          int (*set_group_asid)(struct vdpa_device *vdev, unsigned int group,
>>                                unsigned int asid);
>>          struct device *(*get_vq_dma_dev)(struct vdpa_device *vdev, u16 idx);
>> --
>> 1.8.3.1
>>