On 28/10/2024 21:06, Peter Xu wrote:
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On Mon, Oct 28, 2024 at 07:20:27PM +0200, Avihai Horon wrote:
On 28/10/2024 18:58, Peter Xu wrote:
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On Mon, Oct 28, 2024 at 06:41:42PM +0200, Avihai Horon wrote:
On 28/10/2024 17:45, Peter Xu wrote:
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On Mon, Oct 28, 2024 at 09:43:16AM +0200, Avihai Horon wrote:
On 25/10/2024 0:30, Peter Xu wrote:
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We have two outside users of this API, so it's exported.
Is it really necessary? Does it matter whether it must be
ACTIVE/POSTCOPY_ACTIVE/DEVICE? I guess no.
Actually for VFIO it does matter, because we don't want VFIO to do DPT
log_sync in SETUP stage when DPT might not have been started yet.
See commit ff180c6bd7a8 ("vfio/migration: Skip log_sync during migration
SETUP state").
This seems to be a known issue for migration in general, rather than VFIO
specific. Hyman has a patch for it, not yet reviewed..
https://lore.kernel.org/r/[email protected]
That corresponds to your comment here:
Redundant -- all RAM is marked dirty in migration SETUP state and is
transferred only after migration is set to ACTIVE state, so doing
log_sync during migration SETUP is pointless.
So I wonder whether it's only VFIO that should skip it, or log_sync()
simply shouldn't be called at all during SETUP, because of its redundancy.
Not sure why this sync was there in the first place, but if its only purpose
was to sync dirty pages then yes, I guess it be dropped.
The other thing you mentioned here:
Can fail -- there is a time window, between setting migration state to
SETUP and starting dirty tracking by RAM save_live_setup handler, during
which dirty tracking is still not started. Any VFIO log_sync call that
is issued during this time window will fail. For example, this error can
be triggered by migrating a VM when a GUI is active, which constantly
calls log_sync.
This is VFIO specific. Why this can fail even if global tracking is
started already?
It can fail if global tracking is *not* started yet.
As mentioned in the commit message, there is a time window where migration
is in SETUP state but global tracking is not started yet.
Hmm, I'm totally confused now..
The only thing that can kickoff the sync during SETUP, AFAICT, is:
ret = memory_global_dirty_log_start(GLOBAL_DIRTY_MIGRATION, errp);
if (!ret) {
goto out_unlock;
}
migration_bitmap_sync_precopy(false); <------------- here
I need to confess this may not be the right place to invoke it in ram.c (I
think we probably should move it out at some point.. into generic migration
code). However I don't yet see why log_start() is not called first in your
case before log_sync().
Could you elaborate?
Indeed, in the above code log_start is called before log_sync.
I was referring to the case where some other code path triggers log_sync.
E.g., if you open a VNC to the guest then it constantly calls log_sync to
refresh the graphics. In that case, one of these log_syncs can happen
between "migration status is set to SETUP" and "global tracking is started".
I see. That's unfortunate..
Though this is also the case where it shouldn't be VFIO's problem alone.
See some other users of log_sync():
vhost_sync_dirty_bitmap():
if (!dev->log_enabled || !dev->started) {
return 0;
}
kvm_slot_get_dirty_log():
if (ret == -ENOENT) {
/* kernel does not have dirty bitmap in this slot */
ret = 0;
}
And I didn't further look.
In short, IMHO looks like VFIO still shouldn't be special on differeciating
and make migration export the SETUP phase just for this.. as VFIO has
log_start() like all the rest, so VFIO can also know whether tracking is
enabled at all, then it can silently no-op the log_sync() like all the rest
of the users.
If you agree, I'd prefer we keep this patch - it'll be nice we only ever
expose migration_is_running() for migration status checks, without exposing
SETUP only for this VFIO use case even if it could have followed what other
modules are doing.
Yes, I agree this could be a nice cleanup.
If you would like to propose a patch for VFIO, I'd be happy to include your
patch before this patch (just in case this patch could land some day) to
make sure VFIO works as before. Since I don't have VFIO HW to test, it'll
be challenging for me to propose and test such patch otherwise.
I can do that, though it may be a bit involved because VFIO has multiple
dirty tracking mechanisms (legacy, device DPT and IOMMU DPT). Plus, I
don't have HW that supports IOMMU DPT at hand for testing.
I assume this is not an urgent cleanup, right?
Thanks.
