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

[Michal Privoznik <mprivozn@redhat.com>]

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?

Michal