Re: [Qemu-devel] [PATCH 0/2][RFC] postcopy migration: Linux char device for postcopy

[Avi Kivity <avi@redhat.com>]

On 12/29/2011 03:26 AM, Isaku Yamahata wrote:
> This is Linux kernel driver for qemu/kvm postcopy live migration.
> This is used by qemu/kvm postcopy live migration patch.
>
> TODO:
> - Consider FUSE/CUSE option
>   So far several mmap patches for FUSE/CUSE are floating around. (their
>   purpose isn't different from our purpose, though). They haven't merged
>   into the upstream yet.
>   The driver specific part in qemu patches is modularized. So I expect it
>   wouldn't be difficult to switch kernel driver to CUSE based driver.

It would be good to get more input about this, please involve lkml and
the FUSE/CUSE people.

> ioctl commands:
>
> UMEM_DEV_CRATE_UMEM: create umem device for qemu
> UMEM_DEV_LIST: list created umem devices
> UMEM_DEV_REATTACH: re-attach the created umem device
> 		  UMEM_DEV_LIST and UMEM_DEV_REATTACH are used when
> 		  the process that services page fault disappears or get stack.
> 		  Then, administrator can list the umem devices and unblock
> 		  the process which is waiting for page.

Ah, I asked about this in my patch comments.  I think this is done
better by using SCM_RIGHTS to pass fds along, or asking qemu to launch a
new process.

Introducing a global namespace has a lot of complications attached.

>
> UMEM_GET_PAGE_REQUEST: retrieve page fault of qemu process
> UMEM_MARK_PAGE_CACHED: mark the specified pages pulled from the source
>                        for daemon
>
> UMEM_MAKE_VMA_ANONYMOUS: make the specified vma in the qemu process
> 			 This is _NOT_ implemented yet.
>                          anonymous I'm not sure whether this can be implemented
>                          or not.

How do we find out?  This is fairly important, stuff like transparent
hugepages and ksm only works on anonymous memory.

-- 
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