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

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Message-ID: <CACGkMEscjR_bTVfwaRcQ8qxpiOEJAT35Y1uoj=kBptYkbijDbw@mail.gmail.com>
Date:   Wed, 16 Aug 2023 09:55:33 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Si-Wei Liu <si-wei.liu@...cle.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 Wed, Aug 16, 2023 at 3:49 AM Si-Wei Liu <si-wei.liu@...cle.com> wrote:
>
>
>
> 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.

I meant the bus tries to hide the differences among vendors. So it
needs to hide on-chip IOMMU stuff to the upper layer.

We can expose two dimensional IO mappings models but it looks like
over engineering for this issue. More below.

>
>   * @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.

We can do this simple change if it can work.

>  Further this doesn't work with .dma_map parent drivers.

Probably, but I'd remove dma_map as it doesn't have any real users
except for the simulator.

>
> 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.

I'm ok with a new feature flag, but we need to first seek a way to
reuse the existing API.

Thanks

>
> 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
> >>
>