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



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.

also if ' -machine memory-backend=' is used and '-m' sets only
initial ram size, then '-m' can be omitted as size will be derived
from used backend.

> 
> Michal



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