Re: [RFC V1 00/13] vdpa live update

[Jason Wang]

On Thu, Jan 11, 2024 at 4:40 AM 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 available.  The vdpa device
> descriptor, fd, remains open across the exec.
>
>   ioctl(fd, VHOST_VDPA_SUSPEND)
>   ioctl(fd, VHOST_VDPA_SET_STATUS, 0)
>   exec

Is there a userspace implementation as a reference?

>
>   ioctl(fd, VHOST_NEW_OWNER)
>
>   issue ioctls to re-create vrings
>
>   if VHOST_BACKEND_F_IOTLB_REMAP
>       foreach dma mapping
>           write(fd, {VHOST_IOTLB_REMAP, new_addr})

I think I need to understand the advantages of this approach. For
example, why it is better than

ioctl(VHOST_RESET_OWNER)
exec

ioctl(VHOST_SET_OWNER)

for each dma mapping
     ioctl(VHOST_IOTLB_UPDATE)

Thanks

>
>   ioctl(fd, VHOST_VDPA_SET_STATUS,
>             ACKNOWLEDGE | DRIVER | FEATURES_OK | DRIVER_OK)
>
>
> Steve Sistare (13):
>   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
>   vhost-vdpa: flush workers on suspend
>   vduse: flush workers on suspend
>   vdpa_sim: reset must not run
>   vdpa_sim: flush workers on suspend
>   vdpa/mlx5: new owner capability
>   vdpa_sim: new owner capability
>   vduse: new owner capability
>
>  drivers/vdpa/mlx5/net/mlx5_vnet.c  |   3 +-
>  drivers/vdpa/vdpa_sim/vdpa_sim.c   |  24 ++++++-
>  drivers/vdpa/vdpa_user/vduse_dev.c |  32 +++++++++
>  drivers/vhost/vdpa.c               | 101 +++++++++++++++++++++++++++--
>  drivers/vhost/vhost.c              |  15 +++++
>  drivers/vhost/vhost.h              |   1 +
>  include/uapi/linux/vhost.h         |  10 +++
>  include/uapi/linux/vhost_types.h   |  15 ++++-
>  8 files changed, 191 insertions(+), 10 deletions(-)
>
> --
> 2.39.3
>