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

[Dr. David Alan Gilbert]

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