Re: [Qemu-discuss] Some questions about live migration support on virtio-9p

[Greg Kurz]

On Tue, 29 May 2018 08:11:32 +0000
Linzichang <address@hidden> wrote:

> Hi All,
>          I am using virtio 9pfs in guestOS now, and I’m wonder if live 
> migration supports 9pfs in the latest version of Qemu.

No it is not. There's a migration blocker as long as the 9p file system is
mounted in the guest.

> If not, I would be grateful to know exactly why. Is the device state or guest 
> & host shared memory hard to save?

Yes, sort of. First, I/O requests (or PDUs in 9pfs jargon) are serviced by
a thread pool, and we cannot save their state until they have completed.
This means that we must drain all in-flight PDUs. This isn't too hard. Then,
some enhancements are needed on the VMState side to be able to stream 9pfs
internal structures. Not too hard either. Then we begin to enter the ugly.
Mostly special cases, but they are common enough to be addressed:
- unlinked open files: since the path was unlinked, we cannot re-open it on
  the destination. This could be worked around by creating a temporary file
  on the destination and using the open fd we have on the source to copy
  the content of the original file to the temporary one, re-open it and then
  unlink it.
- files opened with O_ECXL: we cannot re-open the file on the target since
  it already exists, and dropping O_ECXL from the file status flags isn't
  really an option. I couldn't come up with a way to address that yet...

My latest try, about 2 years ago, is available here if you want to look:

https://github.com/gkurz/qemu/commits/9p-migration

> Best regards,
> Clare chen
> 

Cheers,

--
Greg

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