Re: [PATCH V2 0/7] vdpa live update

[Jason Wang]

On Fri, Jul 12, 2024 at 9:19 PM Steve Sistare <steven.sistare@xxxxxxxxxx> wrote:
>
> Live update is a technique wherein an application saves its state, exec's
> to an updated version of itself, and restores its state.  Clients of the
> application experience a brief suspension of service, on the order of
> 100's of milliseconds, but are otherwise unaffected.
>
> Define and implement interfaces that allow vdpa devices to be preserved
> across fork or exec, to support live update for applications such as QEMU.
> The device must be suspended during the update, but its DMA mappings are
> preserved, so the suspension is brief.
>
> The VHOST_NEW_OWNER ioctl transfers device ownership and pinned memory
> accounting from one process to another.
>
> The VHOST_BACKEND_F_NEW_OWNER backend capability indicates that
> VHOST_NEW_OWNER is supported.
>
> The VHOST_IOTLB_REMAP message type updates a DMA mapping with its userland
> address in the new process.
>
> The VHOST_BACKEND_F_IOTLB_REMAP backend capability indicates that
> VHOST_IOTLB_REMAP is supported and required.  Some devices do not
> require it, because the userland address of each DMA mapping is discarded
> after being translated to a physical address.
>
> Here is a pseudo-code sequence for performing live update, based on
> suspend + reset because resume is not yet widely available.  The vdpa device
> descriptor, fd, remains open across the exec.
>
>   ioctl(fd, VHOST_VDPA_SUSPEND)
>   ioctl(fd, VHOST_VDPA_SET_STATUS, 0)

I don't understand why we need a reset after suspend, it looks to me
the previous suspend became meaningless.

>   exec
>
>   ioctl(fd, VHOST_NEW_OWNER)
>
>   issue ioctls to re-create vrings
>
>   if VHOST_BACKEND_F_IOTLB_REMAP

So the idea is for a device that is using a virtual address, it
doesn't need VHOST_BACKEND_F_IOTLB_REMAP at all?

>       foreach dma mapping
>           write(fd, {VHOST_IOTLB_REMAP, new_addr})
>
>   ioctl(fd, VHOST_VDPA_SET_STATUS,
>             ACKNOWLEDGE | DRIVER | FEATURES_OK | DRIVER_OK)

>From API level, this seems to be asymmetric as we have suspending but
not resuming?

>
> This is faster than VHOST_RESET_OWNER + VHOST_SET_OWNER + VHOST_IOTLB_UPDATE,
> as that would would unpin and repin physical pages, which would cost multiple
> seconds for large memories.
>
> This is implemented in QEMU by the patch series "Live update: vdpa"
>   https://lore.kernel.org/qemu-devel/TBD  (reference to be posted shortly)
>
> The QEMU implementation leverages the live migration code path, but after
> CPR exec's new QEMU:
>   - vhost_vdpa_set_owner() calls VHOST_NEW_OWNER instead of VHOST_SET_OWNER
>   - vhost_vdpa_dma_map() sets type VHOST_IOTLB_REMAP instead of
>     VHOST_IOTLB_UPDATE
>
> Changes in V2:
>   - clean up handling of set_map vs dma_map vs platform iommu in remap
>   - augment and clarify commit messages and comments
>
> Steve Sistare (7):
>   vhost-vdpa: count pinned memory
>   vhost-vdpa: pass mm to bind
>   vhost-vdpa: VHOST_NEW_OWNER
>   vhost-vdpa: VHOST_BACKEND_F_NEW_OWNER
>   vhost-vdpa: VHOST_IOTLB_REMAP
>   vhost-vdpa: VHOST_BACKEND_F_IOTLB_REMAP
>   vdpa/mlx5: new owner capability
>
>  drivers/vdpa/mlx5/net/mlx5_vnet.c |   3 +-
>  drivers/vhost/vdpa.c              | 125 ++++++++++++++++++++++++++++--
>  drivers/vhost/vhost.c             |  15 ++++
>  drivers/vhost/vhost.h             |   1 +
>  include/uapi/linux/vhost.h        |  10 +++
>  include/uapi/linux/vhost_types.h  |  15 +++-
>  6 files changed, 161 insertions(+), 8 deletions(-)
>
> --
> 2.39.3
>

Thanks