On Tue, Apr 26, 2016 at 01:14:39PM +1000, Alexey Kardashevskiy wrote: > On 04/19/2016 09:51 PM, Bharata B Rao wrote: > >Memory hotplug can fail for some combinations of RAM and maxmem when > >DDW is enabled in the presence of devices like nec-xhci-usb. DDW depends > >on maximum addressable memory by guest and this value is currently > >being calculated wrongly by the guest kernel routine memory_hotplug_max(). > >While there is an attempt to fix the guest kernel(*), this patch works > >around the problem within QEMU itself. > > > >memory_hotplug_max() routine in the guest kernel arrives at max > >addressable memory by multiplying lmb-size with the lmb-count obtained > >from ibm,dyanmic-memory property. There are two assumptions here: > > > >- All LMBs are part of ibm,dynamic memory: This is not true for PowerKVM > > where only hot-pluggable LMBs are present in this property. > >- The memory area comprising of RAM and hotplug region is contiguous: This > > needn't be true always for PowerKVM as there can be gap between > > boot time RAM and hotplug region. > > > >This work around involves having all the LMBs (RMA, rest of the boot time > >LMBs and hot-pluggable LMBs) as part of ibm,dynamic-memory so that > >guest kernel's calculation of max addressable memory comes out correct > >resulting in correct DDW value which prevents memory hotplug failures. > >memory@0 is created for RMA, but RMA LMBs are also represented as > >"reserved" LMBs in ibm,dynamic-memory. Parts of this are essenitally a > >revert of e8f986fc57a664a74b9f685b466506366a15201b. > > > >In addition to this, the alignment of hotplug memory region is reduced from > >current 1G to 256M (LMB size in PowerKVM) so that we don't end up with any > >gaps between boot time RAM and hotplug region. > > > >This change has a side effect on how the memory nodes in DT are > >represented before and after this change. As an example consider > >a guest with the following memory related command line options: > > > >-m 4G,slots=32,maxmem=8G -numa node,nodeid=0,mem=2G -numa > >node,nodeid=1,mem=2G > > > >Before this change, the guest would have > > > >Scenario 1 > >---------- > >memory@0 for RMA > >memory@80000000 for rest of the boot time memory > >ibm,dynamic-reconfiguration-memory for hot-pluggable memory. > > > >After this commit, the guest will have > > > >Scenario 2 > >---------- > >memory@0 for RMA > >ibm,dynamic-reconfiguration-memory for the entire memory including > >RMA, boot time memory as well as hot-pluggable memory. > > > >If an existing guest having DT nodes as in Scenario 1 above is migrated > >to a QEMU which has this change, at the target, it continues to have the > >DT nodes as in Scenario 1. However after 1st reboot, the DT representation > >changes over to Scenario 2. > > > Does this patch also enable hot-unplug or RMA and boot time LMBs?
No, this patch will not enable hot-unplug for RMA or boot time LMBs. > > Also, this would look nicer on top of "[RFC for-2.7 00/11] A new > infrastructure for guest device trees" where all memory DT nodes are in one > place so it is easier to follow what exactly you... Let me take a look at that patchset. Regards, Bharata.
