Re: [Qemu-devel] [PATCH v2 10/10] hostmem: use object id for memory region name with >= 3.1

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 31 Oct 2018 17:27:57 -0300
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Tue, Oct 30, 2018 at 07:04:53PM +0400, Marc-André Lureau wrote:
> > hostmem-file and hostmem-memfd use the whole object path for the
> > memory region name, and hostname-ram uses only the path component (the
> > object id, or canonical path basename):
> > 
> > qemu -m 1024 -object memory-backend-file,id=mem,size=1G,mem-path=/tmp/foo -numa node,memdev=mem -monitor stdio
> > (qemu) info ramblock
> >               Block Name    PSize              Offset               Used              Total
> >             /objects/mem    4 KiB  0x0000000000000000 0x0000000040000000 0x0000000040000000
> > 
> > qemu -m 1024 -object memory-backend-memfd,id=mem,size=1G -numa node,memdev=mem -monitor stdio
> > (qemu) info ramblock
> >               Block Name    PSize              Offset               Used              Total
> >             /objects/mem    4 KiB  0x0000000000000000 0x0000000040000000 0x0000000040000000
> > 
> > qemu -m 1024 -object memory-backend-ram,id=mem,size=1G -numa node,memdev=mem -monitor stdio
> > (qemu) info ramblock
> >               Block Name    PSize              Offset               Used              Total
> >                      mem    4 KiB  0x0000000000000000 0x0000000040000000 0x0000000040000000
> > 
> > Use the object id for -file and -memfd with >= 3.1 for consistency.
> > Having a consistent naming allow to migrate to different hostmem
> > backends.
> > 
> > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>  
> 
> I don't want to make you feel like you wasted your time on the
> global property system refactor, but:
> 
> Maybe it would be simpler to add a
>   bool MachineClass::canonical_path_for_ramblock_id
I dislike adding compat flags in random places in this case 
it has nothing to do with machine.

> field, instead of refactoring the global property system,
> considering that we're past soft freeze?
We can merge it for 3.2,
I have another potential usage for Mark's work,
converting initial memory to memory devices (so far only ARM),
where we would need to keep ramblock id's compatible across
different machine versions and a different way we were creating
initial memory memory regions so migration would not fail.

> Sometimes I think the global property system was a mistake, and
> that we should avoid spreading it to other subsystems.
Well user accessible globals might be a mistake,
but it's quite useful/convenient API to fix compatibility parameters
for different machines/versions in uniform way without hacking
unrelated code.