Re: [libvirt] [PATCH 3/5] qemu: prefer memfd for anonymous memory

* Marc-André Lureau (marcandre.lureau(a)redhat.com) wrote: > Hi > > On Tue, Sep 11, 2018 at 2:32 PM, Dr. David Alan Gilbert > <dgilbert(a)redhat.com&gt; wrote: > > * Marc-André Lureau (marcandre.lureau(a)redhat.com) wrote: > >> Hi > >> > >> On Tue, Sep 11, 2018 at 12:37 PM, Michal Privoznik <mprivozn(a)redhat.com&gt; wrote: > >> > On 09/11/2018 12:46 AM, John Ferlan wrote: > >> >> > >> >> On 09/07/2018 07:32 AM, marcandre.lureau(a)redhat.com wrote: > >> >>> From: Marc-André Lureau <marcandre.lureau(a)redhat.com&gt; > >> >>> > >> >> > >> >> 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,

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.

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?

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(a)redhat.com / Manchester, UK -- Dr. David Alan Gilbert / dgilbert(a)redhat.com / Manchester, UK