On 03/05/2018 10:09 PM, Michael S. Tsirkin wrote:
On Mon, Mar 05, 2018 at 11:36:15AM +0800, Wei Wang wrote:
On 03/03/2018 02:37 AM, Michael S. Tsirkin wrote:
On Fri, Mar 02, 2018 at 04:47:29PM +0800, Wei Wang wrote:
diff --git a/include/sysemu/balloon.h b/include/sysemu/balloon.h
index af49e19..16a2aae 100644
--- a/include/sysemu/balloon.h
+++ b/include/sysemu/balloon.h
...
+typedef void (QEMUBalloonFreePageStart)(void *opaque);
+typedef void (QEMUBalloonFreePageStop)(void *opaque);
So I think the rule is that no bitmap sync must happen
between these two, otherwise a hint might arrive and
override the sync output.
Should be documented I think.
Yes, agree.
Ideally we'd also detect violations and trigger an assert.
How about just invoking
if (rs->free_page_support)
balloon_free_page_stop();
at the beginning of migration_bitmap_sync()? (balloon_free_page_stop
will just return if the optimization has stopped.)
In this way, we can always have the guarantee that "no bitmap sync must
happen between these two"
How about adding the following new balloon API explanation to
this patch's commit:
- balloon_free_page_start: Callers call this API to obtain guest free
page hints, and clear the related bits from the migration dirty
bitmap.
The whole process is implemented in a new thread independent of the
migration thread. Free page hints imply the part of guest memory is
likely to be free without a guarantee. That is, the reported free
pages
may not be free any more when QEMU receives them, so callers are
responsible for detecting those pages that are not free pages after
the
bits are cleared from the dirty bitmap. To ensure the above, this API
should be used when the migration dirty logging mechanism (e.g.
guest memory write-protection) has started.
- balloon_free_page_stop: Callers call this API to stop the guest from
reporting free page hints. Bits from the dirty bitmap are safe to
be cleared on condition that the dirty logging mechanism is recording
pages that the guest has written to. To avoid the case that clearing
bits of free page hints overrides the dirty bits offered by the dirty
logging mechanism, this API is suggested to be called before QEMU
synchronizes the dirty logging bitmap.
- balloon_free_page_support: This API is called to check whether the
balloon device supports the guest free page reporting feature. The
balloon_free_page_start and balloon_free_page_stop APIs should be used
only when this API returns true.
Best,
Wei
I find this more confusing than explaining.
Let me try
balloon_free_page_start - start guest free page hint reporting.
Note: balloon will report pages which were free at the time
of this call. As the reporting happens asynchronously,
we rely on dirty logging to be started before this call is made.
The dirty logging bitmap must be synchronized before this call
and then after balloon_free_page_stop.
I think it would be better to remove the above one sentence.
I agree "No dirty bitmap synchronizations are allowed between
balloon_free_page_start and balloon_free_page_stop", but "The dirty
logging bitmap MUST be synchronized before balloon_free_page_start"
seems confusing, for example the bulk stage doesn't have to start with a
bitmap sync.
balloon_free_page_stop: stop the guest reporting
of free pages. dirty logging bitmap can be synchronized
after this point.
No bitmap synchronizations are allowed between these two points.
Best,
Wei
