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



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 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. 


Regards,
Daniel
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