Re: [PATCH REPOST v3 78/80] hostmem: fix strict bind policy

[Halil Pasic]

On Mon, 27 Jan 2020 08:39:25 +0100
Igor Mammedov <address@hidden> wrote:

> On Fri, 24 Jan 2020 20:17:48 +0100
> Halil Pasic <address@hidden> wrote:
> 
> > On Thu, 23 Jan 2020 12:38:43 +0100
> > Igor Mammedov <address@hidden> wrote:
> > 
> > > With main RAM now converted to hostmem backends, there is no
> > > point in keeping global mem_prealloc around, so alias
> > >  -mem-prealloc to "memory-backend.prealloc=on"
> > > machine compat[*] property and make mem_prealloc a local
> > > variable to only stir registration of compat property.
> > > 
> > > *) currently user accessible -global works only with DEVICE
> > >    based objects and extra work is needed to make it work
> > >    with hostmem backends. But that is convenience option
> > >    and out of scope of this already huge refactoring.
> > >    Hence machine compat properties were used.  
> > 
> > AFAIU because of this something like
> > -global memory-backend-file.share=on
> > (as proposed by
> > https://lists.nongnu.org/archive/html/qemu-devel/2019-12/msg00531.html)
> > can not be used to make the main RAM shared (e.g. for vhost on s390x).
> > Or am I wrong? If not, is -global still the way we want to make this work
> > for non-numa machines, or did I miss updates?
> 
> one should be able to use memory-backend property to make it work
> instead of -m convenience option in s390 case.

Thank you very much for the quick response!

Honestly, I overlooked the memory-backed machine property, but regardless
of that -machine,memory-backend=id *does not seem viable* at the
moment.

My understanding is that one has to do something like:        
-machine type=s390-ccw-virtio,memory-backend=mem \
-object 
memory-backend-file,id=mem,size=2G,mem-path=/dev/shm/virtiofs.shm,share=on \

I get 
qemu: : Device 'mem' not found
because the 'memory-backend-*' objects are delayed,
i.e. !object_create_initial(), and at the time when
machine_set_property() tries to look the memory-backend up the
memory-backend is not yet created.

For why delayed, object_create_initial()  has a comment:

    /* Memory allocation by backends needs to be done
     * after configure_accelerator() (due to the tcg_enabled()
     * checks at memory_region_init_*()).
     *
     * Also, allocation of large amounts of memory may delay
     * chardev initialization for too long, and trigger timeouts
     * on software that waits for a monitor socket to be created
     * (e.g. libvirt).
     */
    if (g_str_has_prefix(type, "memory-backend-")) {

Or, am I using it wrong?

> 
> As for -global for objects (or more limited variant for memory-backends),
> it needs more work to support objects. (that's mostly policy decision)
> 

I agree.

Regards,
Halil