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)
