> 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. Then we should pause the whole VM when using the bitmap, too expensive? > 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. > How about rename the interface to a more appropriate name other than 'free page' ? Liang. > 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
