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

[Marc-André Lureau]

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

>
> 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