On Fri, Apr 22, 2016 at 10:48:38AM +0100, Dr. David Alan Gilbert wrote: > * Michael S. Tsirkin (mst@xxxxxxxxxx) wrote: > > On Tue, Apr 19, 2016 at 03:02:09PM +0000, Li, Liang Z wrote: > > > > On Tue, 2016-04-19 at 22:34 +0800, Liang Li wrote: > > > > > The free page bitmap will be sent to QEMU through virtio interface and > > > > > used for live migration optimization. > > > > > Drop the cache before building the free page bitmap can get more free > > > > > pages. Whether dropping the cache is decided by user. > > > > > > > > > > > > > How do you prevent the guest from using those recently-freed pages for > > > > something else, between when you build the bitmap and the live migration > > > > completes? > > > > > > Because the dirty page logging is enabled before building the bitmap, there is no need > > > to prevent the guest from using the recently-freed pages ... > > > > > > Liang > > > > Well one point of telling host that page is free is so that > > it can mark it clean even if it was dirty previously. > > So I think you must pass the pages to guest under the lock. > > This will allow host optimizations such as marking these > > pages MADV_DONTNEED or MADV_FREE. > > Otherwise it's all too tied up to a specific usecase - > > you aren't telling host that a page is free, you are telling it > > that a page was free in the past. > > But doing it under lock sounds pretty expensive, especially given > how long the userspace side is going to take to work through the bitmap > and device what to do. > > Dave We need to make it as fast as we can since the VCPU is stopped on exit anyway. This just means e.g. sizing the bitmap reasonably - don't always try to fit all memory in a single bitmap. Really, if the page can in fact be in use when you tell host it's free, then it's rather hard to explain what does it mean from host/guest interface point of view. It probably can be defined but the interface seems very complex. Let's start with a simple thing instead unless it can be shown that there's a performance problem. > > > > -- > > MST > -- > Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
