[PATCH REBASE 7/7] qemu: Use memory-backend-* for regular guest memory

[Michal Privoznik]

On 9/15/20 11:59 AM, Daniel P. Berrangé wrote:
> On Tue, Sep 15, 2020 at 11:53:56AM +0200, Igor Mammedov wrote:
>> On Tue, 15 Sep 2020 10:54:46 +0200
>> Michal Privoznik <mprivozn at redhat.com> wrote:
>>
>>> On 9/8/20 3:55 PM, Ján Tomko wrote:
>>>> On a Tuesday in 2020, Michal Privoznik wrote:
>>>
>>>>> diff --git
>>>>> a/tests/qemuxml2argvdata/memfd-memory-default-hugepage.x86_64-latest.args
>>>>> b/tests/qemuxml2argvdata/memfd-memory-default-hugepage.x86_64-latest.args
>>>>> index 5d256c42bc..b43e7d9c3c 100644
>>>>> ---
>>>>> a/tests/qemuxml2argvdata/memfd-memory-default-hugepage.x86_64-latest.args
>>>>> +++
>>>>> b/tests/qemuxml2argvdata/memfd-memory-default-hugepage.x86_64-latest.args
>>>>> @@ -12,14 +12,16 @@ QEMU_AUDIO_DRV=none \
>>>>> -S \
>>>>> -object secret,id=masterKey0,format=raw,\
>>>>> file=/tmp/lib/domain--1-instance-00000092/master-key.aes \
>>>>> --machine pc-i440fx-2.3,accel=kvm,usb=off,dump-guest-core=off \
>>>>> +-machine pc-i440fx-2.3,accel=kvm,usb=off,dump-guest-core=off,\
>>>>> +memory-backend=pc.ram \
>>>>> -cpu qemu64 \
>>>>> -m 14336 \
>>>>> --mem-prealloc \
>>>>> +-object memory-backend-memfd,id=pc.ram,hugetlb=yes,hugetlbsize=2097152,\
>>>>> +share=yes,prealloc=yes,size=15032385536 \
>>>>> -overcommit mem-lock=off \
>>>>> -smp 8,sockets=1,dies=1,cores=8,threads=1 \
>>>>> -object
>>>>> memory-backend-memfd,id=ram-node0,hugetlb=yes,hugetlbsize=2097152,\
>>>>> -share=yes,size=15032385536,host-nodes=3,policy=preferred \
>>>>> +share=yes,prealloc=yes,size=15032385536,host-nodes=3,policy=preferred \
>>>>> -numa node,nodeid=0,cpus=0-7,memdev=ram-node0 \
>>>>> -uuid 126f2720-6f8e-45ab-a886-ec9277079a67 \
>>>>> -display none \
>>>>
>>>> Should we format all the fields twice in these cases?
>>>
>>> Ah, good question. Honestly, I don't remember, it was slightly longer
>>> ago that I've written these patches. Igor, do you perhaps remember
>>> whether libvirt needs to specify both: -machine memory-backend=$id and
>>> -object memory-backend-*,id=$id?
>>
>> the later defines backend and the former uses it,
>> short answer is yes.
>>
>> you do not need
>>   --mem-prealloc
>> if you explicitly set "prealloc=yes" on backend.
>>
>> I'd prefer if libvirt stopped using old -mem-prealloc and -mem-path
>> in favor of explicit properties on backend, so QEMU could deprecate
>> it and drop aliasing code which uses global properties hack.
> 
> IIRC, we tried todo that in the past and the change to use a backend
> impacted migration ABI compatibility.

Yes, but the problem there was that the memory IDs were not the same. 
That is: -m + -mem-path would create a memory-backend-file equivalent 
and set its ID to pc.ram (assuming this is x86 - hence default-ram-id 
exposure and tracking). But libvirt did generate different ID back then 
which was incompatible.

Michal