[PATCH v2 0/4] vdpa: decouple reset of iotlb mapping from device reset

[Si-Wei Liu]

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 based off of the descriptor group v3 series
from Dragos. [2]

[1] Reducing vdpa migration downtime because of memory pin / maps
https://www.mail-archive.com/qemu-devel@xxxxxxxxxx/msg953755.html
[2] [PATCH vhost v3 00/16] vdpa: Add support for vq descriptor mappings
https://lore.kernel.org/lkml/20231009112401.1060447-1-dtatulea@xxxxxxxxxx/

---
v2:
- improved commit message to clarify the intended csope of .reset_map API
- improved commit messages to clarify no breakage on older userspace

v1:
- rewrote commit messages to include more detailed description and background
- reword to vendor specific IOMMU implementation from on-chip IOMMU
- include parent device backend feautres to persistent iotlb precondition
- reimplement mlx5_vdpa patch on top of descriptor group series

RFC v3:
- fix missing return due to merge error in patch #4

RFC v2:
- rebased on top of the "[PATCH RFC v2 0/3] vdpa: dedicated descriptor table group" series:
  https://lore.kernel.org/virtualization/1694248959-13369-1-git-send-email-si-wei.liu@xxxxxxxxxx/

---

Si-Wei Liu (4):
  vdpa: introduce .reset_map operation callback
  vhost-vdpa: reset vendor specific mapping to initial state in .release
  vhost-vdpa: introduce IOTLB_PERSIST backend feature bit
  vdpa/mlx5: implement .reset_map driver op

 drivers/vdpa/mlx5/core/mlx5_vdpa.h |  1 +
 drivers/vdpa/mlx5/core/mr.c        | 17 ++++++++++++++++
 drivers/vdpa/mlx5/net/mlx5_vnet.c  | 18 ++++++++++++-----
 drivers/vhost/vdpa.c               | 31 ++++++++++++++++++++++++++++++
 include/linux/vdpa.h               | 10 ++++++++++
 include/uapi/linux/vhost_types.h   |  2 ++
 6 files changed, 74 insertions(+), 5 deletions(-)

-- 
2.39.3