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Subject: Re: [PATCH v3 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT
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 freepage 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 islikely to be free without a guarantee. That is, the reported free pages
may not be free any more when QEMU receives them, so callers areresponsible 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. Theballoon_free_page_start and balloon_free_page_stop APIs should be used
only when this API returns true. Best, Wei
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