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

[Michal Privoznik <mprivozn@xxxxxxxxxx>]

On 9/15/20 12:07 PM, Michal Privoznik wrote:
> 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@xxxxxxxxxx> 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.

I remembered this discussion happening but couldn't find anywhere on the 
list. Of course, because it happened in bugzilla:

https://bugzilla.redhat.com/show_bug.cgi?id=1836043#c4

Michal