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

[Steven Sistare <steven.sistare@oracle.com>]

On 7/12/2024 9:18 AM, Steve Sistare 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)
>    exec
> 
>    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})
> 
>    ioctl(fd, VHOST_VDPA_SET_STATUS,
>              ACKNOWLEDGE | DRIVER | FEATURES_OK | DRIVER_OK)
> 
> 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)

https://lore.kernel.org/qemu-devel/1720792931-456433-3-git-send-email-steven.sistare@oracle.com

> 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(-)
>