Re: [PATCH v2] machine: add missing doc for memory-backend option

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 15 Jan 2021 10:36:05 +0100
Michal Privoznik <mprivozn@redhat.com> wrote:

> On 1/15/21 12:46 AM, Igor Mammedov wrote:
> > Add documentation for '-machine memory-backend' CLI option and
> > how to use it.
> > 
> > And document that x-use-canonical-path-for-ramblock-id,
> > is considered to be stable to make sure it won't go away by accident.
> > 
> > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > ---
> > v2:
> >   - add doc that x-use-canonical-path-for-ramblock-id is considered stable,
> >     (Peter Krempa <pkrempa@redhat.com>)
> > ---
> >   backends/hostmem.c | 10 ++++++++++
> >   qemu-options.hx    | 28 +++++++++++++++++++++++++++-
> >   2 files changed, 37 insertions(+), 1 deletion(-)
> >   
> 
> 
> > @@ -96,6 +97,31 @@ SRST
> >       ``hmat=on|off``
> >           Enables or disables ACPI Heterogeneous Memory Attribute Table
> >           (HMAT) support. The default is off.
> > +
> > +     ``memory-backend='id'``
> > +        An alternative to legacy ``-mem-path`` and ``mem-prealloc`` options.
> > +        Allows to use a memory backend as main RAM.
> > +
> > +        For example:
> > +        ::
> > +        -object memory-backend-file,id=pc.ram,size=512M,mem-path=/hugetlbfs,prealloc=on,share=on
> > +        -machine memory-backend=pc.ram
> > +        -m 512M
> > +
> > +        Migration compatibility note:
> > +        a) as backend id one shall use value of 'default-ram-id', advertised by
> > +        machine type (available via ``query-machines`` QMP command)
> > +        b) for machine types 4.0 and older, user shall
> > +        use ``x-use-canonical-path-for-ramblock-id=on`` backend option
> > +        (this option must be considered stable, as if it didn't have the 'x-'
> > +        prefix including deprecation period, as long as 4.0 and older machine
> > +        types exists),
> > +        if migration to/from old QEMU (<5.0) is expected.
> > +        For example:
> > +        ::
> > +        -object memory-backend-ram,id=pc.ram,size=512M,x-use-canonical-path-for-ramblock-id=on
> > +        -machine memory-backend=pc.ram
> > +        -m 512M  
> 
> Igor, this doesn't correspond with your comment in bugzilla:
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1836043#c31
> 
> In fact, we had to turn the attribute OFF so that canonical path is not 
> used. Isn't ON the default state anyway?

indeed, it should be 'off',

> 
> Michal
> 
>