Hi, I think they was already reported some month ago, and a patch was submitted to the mailing list (but waiting that memory unplug was merged before apply it) http://lists.gnu.org/archive/html/qemu-devel/2014-11/msg02362.html ----- Mail original ----- De: "Luiz Capitulino" <lcapitulino@xxxxxxxxxx> À: "qemu-devel" <qemu-devel@xxxxxxxxxx> Cc: "kvm" <kvm@xxxxxxxxxxxxxxx>, "Igor Mammedov" <imammedo@xxxxxxxxxx>, "zhang zhanghailiang" <zhang.zhanghailiang@xxxxxxxxxx>, pkrempa@xxxxxxxxxx, "Eric Blake" <eblake@xxxxxxxxxx>, "Michael S. Tsirkin" <mst@xxxxxxxxxx>, "amit shah" <amit.shah@xxxxxxxxxx> Envoyé: Jeudi 26 Février 2015 20:26:29 Objet: [BUG] Balloon malfunctions with memory hotplug Hello, Reproducer: 1. Start QEMU with balloon and memory hotplug support: # qemu [...] -m 1G,slots=2,maxmem=2G -balloon virtio 2. Check balloon size: (qemu) info balloon balloon: actual=1024 (qemu) 3. Hotplug some memory: (qemu) object_add memory-backend-ram,id=mem1,size=1G (qemu) device_add pc-dimm,id=dimm1,memdev=mem1 4. This is step is _not_ needed to reproduce the problem, but you may need to online memory manually on Linux so that it becomes available in the guest 5. Check balloon size again: (qemu) info balloon balloon: actual=1024 (qemu) BUG: The guest now has 2GB of memory, but the balloon thinks the guest has 1GB One may think that the problem is that the balloon driver is ignoring hotplugged memory. This is not what's happening. If you do balloon your guest, there's nothing stopping the balloon driver in the guest from ballooning hotplugged memory. The problem is that the balloon device in QEMU needs to know the current amount of memory available to the guest. Before memory hotplug this information was easy to obtain: the current amount of memory available to the guest is the memory the guest was booted with. This value is stored in the ram_size global variable in QEMU and this is what the balloon device emulation code uses today. However, when memory is hotplugged ram_size is _not_ updated and the balloon device breaks. I see two possible solutions for this problem: 1. In addition to reading ram_size, the balloon device in QEMU could scan pc-dimm devices to account for hotplugged memory. This solution was already implemented by zhanghailiang: http://lists.gnu.org/archive/html/qemu-devel/2014-11/msg02362.html It works, except that on Linux memory hotplug is a two-step procedure: first memory is inserted then it has to be onlined from user-space. So, if memory is inserted but not onlined this solution gives the opposite problem: the balloon device will report a larger memory amount than the guest actually has. Can we live with that? I guess not, but I'm open for discussion. If QEMU could be notified when Linux makes memory online, then the problem would be gone. But I guess this can't be done. 2. Modify the balloon driver in the guest to inform the balloon device on the host about the current memory available to the guest. This way, whenever the balloon device in QEMU needs to know the current amount of memory in the guest, it asks the guest. This drops any usage of ram_size in the balloon device I'm not completely sure this is feasible though. For example, what happens if the guest reports a memory amount to QEMU and right after this more memory is plugged? Besides, this solution is more complex than solution 1 and won't address older guests. Another important detail is that, I *suspect* that a very similar bug already exists with 32-bit guests even without memory hotplug: what happens if you assign 6GB to a 32-bit without PAE support? I think the same problem we're seeing with memory hotplug will happen and solution 1 won't fix this, although no one seems to care about 32-bit guests... -- 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 -- 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
