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From:   Jason Wang <jasowang@redhat.com>
Date:   Mon, 23 Oct 2023 11:51:19 +0800
Message-ID: <CACGkMEvkSFcHXC0HFw-NoDtDNnaucJbpfPO0Yho2r1QP8F6zSw@mail.gmail.com>
Subject: Re: [PATCH v4 0/7] vdpa: decouple reset of iotlb mapping from device reset
To:     Si-Wei Liu <si-wei.liu@oracle.com>
Cc:     mst@redhat.com, eperezma@redhat.com, sgarzare@redhat.com,
        dtatulea@nvidia.com, virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
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Hi Si-Wei:

On Sat, Oct 21, 2023 at 5:28=E2=80=AFPM Si-Wei Liu <si-wei.liu@oracle.com> =
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@nongnu.org/msg953755.html
>
> ---
> v4:
> - Rework compatibility using new .compat_reset driver op

I still think having a set_backend_feature() or reset_map(clean=3Dtrue)
might be better. As it tries hard to not introduce new stuff on the
bus.

But we can listen to others for sure.

Thanks