Fabrice Bellard wrote:
Anthony Liguori wrote:
Howdy,
I've been working on migration for QEMU and have run into a snag.
I've got a non-live migration patch that works quite happily[1]. I
modified the save/restore code to not seek at all, and then basically
pipe a save over a pipe to a subprocess (usually, ssh).
qumranet has written some code to do live migration too.
Yes, I looked at their code before starting. They don't currently do
live migration (only offline migration which is really just a
save/restore over the network). I found their implementation to be a
bit overly complicated so I decided to try my own.
IMHO, client/server code should be integrated in QEMU in order to ease
the use of live migration.
Agreed. What I'd like to eventually have is a migrate command that took
a URL. For instance:
(qemu) migrate ssh://woolly
(qemu) migrate tcp://woolly:8001
While still maintaining the ability to specific a full ssh command.
This is useful if for many things (like changing to location of disks in
case you mount an NFS partition in different locations). While I see
the value in a TCP transport, I personally want an SSH transport so I
don't to bother setting up a server.
The current code is just something that works.
Can anyone provide me with some advice on how to do this? Am I right
in assuming that all IO will go through some function?
RAM access is not handled via I/O for efficiency, but the
phys_ram_dirty flags are always up to date. In order to use it, you
must allocate one bit in the dirty flags not used by QEMU and kqemu.
Then you can use:
cpu_physical_memory_reset_dirty() to mark a page as not dirty and
cpu_physical_memory_get_dirty() to test for dirtiness.
I didn't realize that you could reset an individual bit and it would be
updated appropriately--but it definitely works. This simplifies things
a lot. Thanks!
Note that for performance reasons the dirty bits are not handled while
QEMU modifies the A and D bits in the PTEs and it can be a problem for
your application.
Right, I saw that. For now, I've modified the stl_phys_notdirty()
function to update the dirty bitmap iff there is an active migration
going on. I can wrap this check in an unexpected() so there should be
very little performance impact.
I've noticed that while this works fine for QEMU and user KQEMU, it
doesn't work with kernel KQEMU. I suspect this is because kernel KQEMU
is updating A/D bits without updating the dirty bitmap. Hopefully you
could fix that once we get migration working.
FYI, the dirty bits are currently used in QEMU to optimize VGA
refreshs and to track self modifying code. They are also used
internally by kqemu.
Does KQEMU use bits other than 1,2 (VGA_DIRTY_FLAG, CODE_DIRTY_FLAG)?
I've added a MIGRATION_DIRTY_FLAG as bit 3 and there doesn't seem to be
a conflict.
Thanks again Fabrice,
Anthony Liguori
Regards,
Fabrice.
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