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Re: [Qemu-devel] [PATCH v3 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PA
From: |
Wei Wang |
Subject: |
Re: [Qemu-devel] [PATCH v3 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT |
Date: |
Mon, 05 Mar 2018 11:36:15 +0800 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 |
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. 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