Re: [PATCH v4 0/7] vdpa: decouple reset of iotlb mapping from device reset

[Jason Wang]

Hi Si-Wei:

On Sat, Oct 21, 2023 at 5:28 PM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> wrote:
>
> In order to reduce needlessly high setup and teardown cost
> of iotlb mapping during live migration, it's crucial to
> decouple the vhost-vdpa iotlb abstraction from the virtio
> device life cycle, i.e. iotlb mappings should be left
> intact across virtio device reset [1]. For it to work, the
> on-chip IOMMU parent device could implement a separate
> .reset_map() operation callback to restore 1:1 DMA mapping
> without having to resort to the .reset() callback, the
> latter of which is mainly used to reset virtio device state.
> This new .reset_map() callback will be invoked only before
> the vhost-vdpa driver is to be removed and detached from
> the vdpa bus, such that other vdpa bus drivers, e.g.
> virtio-vdpa, can start with 1:1 DMA mapping when they
> are attached. For the context, those on-chip IOMMU parent
> devices, create the 1:1 DMA mapping at vdpa device creation,
> and they would implicitly destroy the 1:1 mapping when
> the first .set_map or .dma_map callback is invoked.
>
> This patchset is rebased on top of the latest vhost tree.
>
> [1] Reducing vdpa migration downtime because of memory pin / maps
> https://www.mail-archive.com/qemu-devel@xxxxxxxxxx/msg953755.html
>
> ---
> v4:
> - Rework compatibility using new .compat_reset driver op

I still think having a set_backend_feature() or reset_map(clean=true)
might be better. As it tries hard to not introduce new stuff on the
bus.

But we can listen to others for sure.

Thanks