* Marc-André Lureau (marcandre.lureau@xxxxxxxxxx) wrote: > Hi > > On Tue, Sep 11, 2018 at 3:26 PM, Dr. David Alan Gilbert > <dgilbert@xxxxxxxxxx> wrote: > > * Marc-André Lureau (marcandre.lureau@xxxxxxxxxx) wrote: > >> Hi > >> > >> On Tue, Sep 11, 2018 at 2:32 PM, Dr. David Alan Gilbert > >> <dgilbert@xxxxxxxxxx> wrote: > >> > * Marc-André Lureau (marcandre.lureau@xxxxxxxxxx) wrote: > >> >> Hi > >> >> > >> >> On Tue, Sep 11, 2018 at 12:37 PM, Michal Privoznik <mprivozn@xxxxxxxxxx> wrote: > >> >> > On 09/11/2018 12:46 AM, John Ferlan wrote: > >> >> >> > >> >> >> On 09/07/2018 07:32 AM, marcandre.lureau@xxxxxxxxxx wrote: > >> >> >>> From: Marc-André Lureau <marcandre.lureau@xxxxxxxxxx> > >> >> >>> > >> >> >> > >> >> >> Would be nice to have a few more words here. If you provide them I can > >> >> >> add them... The if statement is difficult to read unless you know what > >> >> >> each field really means. > >> >> >> > >> >> >> secondary question - should we document what gets used?, e.g.: > >> >> >> > >> >> >> https://libvirt.org/formatdomain.html#elementsMemoryBacking > >> >> >> > >> >> >> Seems to me the preference to use memfd is for memory backing using > >> >> >> anonymous source for nvdimm's without a defined path, but sometimes my > >> >> >> wording doesn't match reality. > >> >> > > >> >> > I don't think we want to tell users what backend are we going to use > >> >> > under what conditions. Firstly, these conditions will change (as they > >> >> > did in the past). Secondly, what backend libvirt decides to use is no > >> >> > business of users. I mean, they care about providing XML that matches > >> >> > their demands. It's libvirt's job to fulfil them. > >> >> > > >> >> > Look at this from the other way: if an user wants to have > >> >> > memory-backend-file for his domain, how would they enforce it once memfd > >> >> > is merged? Sure, they can tweak their memoryBacking settings, but that > >> >> > would work only until we decide to change the decision process for mem > >> >> > backend. > >> >> > > >> >> > What I am more worried about is migration. What happens if I migrate a > >> >> > hugepages domain from older libvirt to a newer one (the former doesn't > >> >> > support memfd, the latter does). On the source the domain was started > >> >> > with memory-backend-file (or memory-backend-ram with -mem-path). And > >> >> > during migration, the generated cmd line would use memfd. And I don't > >> >> > think qemu is capable of dealing with this discrepancy, is it? > >> >> > >> >> > >> >> Actually, qemu doesn't care about the hostmem backend kind, it should > >> >> handle the migration ok. > >> >> > >> >> However, there seems to be a bug in qemu, and hostmem backend don't > >> >> use the right qom object name. > >> > > >> > Can you give me the command lines you're using? > >> > >> qemu -m 4096 -object memory-backend-ram,id=mem,size=4G -numa > >> node,memdev=mem -monitor stdio > >> qemu -m 4096 -object > >> memory-backend-file,id=mem,size=4G,mem-path=/tmp/foo -numa > >> node,memdev=mem -monitor stdio > >> qemu -m 4096 -object memory-backend-memfd,id=mem,size=4G -numa > >> node,memdev=mem -monitor stdio > > > > There seem to be two different problems (at least); there's that > > escaping problem where the /'s are shown as \x2f in into qom-tree, > > That's not a problem, this is done in memory_region_escape_name() > > > but info ramblock looks saner, but is still showing the difference: > > > > ./x86_64-softmmu/qemu-system-x86_64 -m 1024 -object memory-backend-ram,id=mem,size=1G -numa node,memdev=mem -monitor stdio > > (qemu) info ramblock > > Block Name PSize Offset Used Total > > mem 4 KiB 0x0000000000000000 0x0000000040000000 0x0000000040000000 > > > > ./x86_64-softmmu/qemu-system-x86_64 -m 1024 -object memory-backend-file,id=mem,size=1G,mem-path=/tmp/foo -numa node,memdev=mem -monitor stdio > > (qemu) info ramblock > > Block Name PSize Offset Used Total > > /objects/mem 4 KiB 0x0000000000000000 0x0000000040000000 0x0000000040000000 > > > > ./x86_64-softmmu/qemu-system-x86_64 -m 1024 -object memory-backend-memfd,id=mem,size=1G -numa node,memdev=mem -monitor stdio > > QEMU 3.0.50 monitor - type 'help' for more information > > (qemu) info ramblock > > Block Name PSize Offset Used Total > > /objects/mem 4 KiB 0x0000000000000000 0x0000000040000000 0x0000000040000000 > > > > hostmem-file.c is using object_get_canonical_path to get the RAMBlock > > where as hostmem-ram.c is using object_get_canonical_path_**component** > > > > The problem is if we change either of them then again we break > > migration compatibility. > > Yes, that was the object of my question :) > > > We could wire it to a machine type and/or property, so that > > memory-backend-ram would use the long name on newere qemus with an > > appropriate flag? > > Good idea, I can prepare a patch. Great; if you add the property to use the longname, then turn that property on in the newer machine type it should work. A qemu that has the property can then be assumed to the right thing when set. > However, libvirt will have to learn of this migration issue with older > version, it's probably not worth to try to make more workarounds. Yeh I'm not sure what your heuristics look like for these choices. But for a VM without this fix then you can't convert from backend-ram to memfd. Dave > > > Dave > > > > > > > >> > > >> > Dave > >> > > >> >> with memory-backend-ram: > >> >> > >> >> (qemu) info qom-tree /objects > >> >> /objects (container) > >> >> /mem (memory-backend-file) > >> >> /mem[0] (qemu:memory-region) > >> >> > >> >> But with memory-backend-file or memory-backend-memfd: > >> >> > >> >> (qemu) info qom-tree /objects > >> >> /objects (container) > >> >> /mem (memory-backend-file) > >> >> /\x2fobjects\x2fmem[0] (qemu:memory-region) > >> >> > >> >> > >> >> This causes migration to fail because of the object naming mismatch. > >> >> > >> >> It can migrate from/to -file and -memfd, since they use the same > >> >> "broken" name, but not with -ram. > >> >> > >> >> I don't know how we can solve this migration issue without breaking > >> >> things further. Any idea David? > >> >> > >> >> > Or is memfd going to be used only for hugepages + <source > >> >> > type='anonymous'/> case (which is not allowed now and thus migration > >> >> > scenario I'm describing can't happen)? > >> >> > >> >> With those patches, memfd is used for anonymous memory (shared or not, > >> >> hpt or not) with an explicit numa configuration. > >> >> > >> >> thanks > >> > -- > >> > Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK > > -- > > Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
