Re: [RFC V1 0/4] iommufd live update

[Jason Gunthorpe <jgg@ziepe.ca>]

On Sat, Jul 20, 2024 at 11:56:40AM -0700, Steve Sistare wrote:
> Live update is a technique wherein an application saves its state, launches
> 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 the IOMMU_IOAS_CHANGE_PROCESS ioctl to allow management and use
> of an iommufd device to be transferred from one process to another.  The
> application is responsible for transferring the device descriptor to the new
> process, eg either by preservation across fork and exec or via SCM_RIGHTS.
> 
> All memory objects that were mapped for iommufd DMA in the old process
> must also be mapped in the new process, but they may have different virtual
> addresses in the new.  The application passes an array of new addresses to
> IOMMU_IOAS_CHANGE_PROCESS, and it atomically updates everything to the current
> process, grabbing the new mm, transferring pinned page counts, and recording
> new virtual addresses.
> 
> If the application directly exec's the new version of itself, it must take
> special care if the mappings may be accessed by kernel threads, such as by
> vfio mdevs.  The original process must fork an identical caretaker
> process which calls VFIO_IOAS_CHANGE_PROCESS to take temporary ownership
> of the mappings without changing VA's.  The original process then exec's
> its successor, which calls VFIO_IOAS_CHANGE_PROCESS with new VA's to take
> ownership from the caretaker.  
> 
> Thanks to Jason Gunthorpe for code and ideas in this series.
> 
> This is implemented in QEMU by the patch series "Live update: iommufd"
>   https://lore.kernel.org/qemu-devel   (to be posted shortly)
> 
> Steve Sistare (4):
>   iommufd: Export do_update_pinned
>   iommufd: Lock all objects
>   iommufd: Add IOMMU_IOAS_CHANGE_PROCESS
>   iommufd: update VA

Oh, this should also be covered in the iommufd selftests..

Jason