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

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

> > 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