Re: [PATCH RESEND] hostmem: Don't report pmem attribute if unsupported

[John Snow]

On 2/16/21 5:23 PM, Eduardo Habkost wrote:
> On Tue, Jan 26, 2021 at 08:48:25AM +0100, Michal Privoznik wrote:
> > When management applications (like Libvirt) want to check whether
> > memory-backend-file.pmem is supported they can list object
> > properties using 'qom-list-properties'. However, 'pmem' is
> > declared always (and thus reported always) and only at runtime
> > QEMU errors out if it was built without libpmem (and thus can not
> > guarantee write persistence). This is suboptimal since we have
> > ability to declare attributes at compile time.
> >
> > Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1915216
> > Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
> > Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> > Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> > Reviewed-by: Igor Mammedov <imammedo@redhat.com>
>
> I'm not a fan of making availability of properties conditional
> (even if at compile time), but if this helps libvirt I guess it
> makes sense.

Compile time might be OK, but if we want to describe everything via QAPI 
eventually, we just need to be able to describe that compile-time 
requisite appropriately.

Conditional at run-time is I think the thing that absolutely has to go 
wherever it surfaces.

> CCing John, who has been thinking a lot about these questions.

Thanks for the heads up. Good reminder that libvirt uses the existence 
of properties as a bellwether for feature support. I don't think I like 
that idea, but I like breaking libvirt even less.

--js

> I'm queueing this on machine-next.  Thanks, and sorry for the delay!
>
> > ---
> >
> > This is just a resend of a patch I've sent earlier with Reviewed-by and
> > Tested-by added:
> >
> > https://lists.nongnu.org/archive/html/qemu-devel/2021-01/msg04558.html
> >
> >   backends/hostmem-file.c | 13 ++++---------
> >   1 file changed, 4 insertions(+), 9 deletions(-)
> >
> > diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
> > index 40e1e5b3e3..7e30eb5985 100644
> > --- a/backends/hostmem-file.c
> > +++ b/backends/hostmem-file.c
> > @@ -123,6 +123,7 @@ static void file_memory_backend_set_align(Object *o, 
> > Visitor *v,
> >       fb->align = val;
> >   }
> >   
> > +#ifdef CONFIG_LIBPMEM
> >   static bool file_memory_backend_get_pmem(Object *o, Error **errp)
> >   {
> >       return MEMORY_BACKEND_FILE(o)->is_pmem;
> > @@ -139,17 +140,9 @@ static void file_memory_backend_set_pmem(Object *o, bool 
> > value, Error **errp)
> >           return;
> >       }
> >   
> > -#ifndef CONFIG_LIBPMEM
> > -    if (value) {
> > -        error_setg(errp, "Lack of libpmem support while setting the 'pmem=on'"
> > -                   " of %s. We can't ensure data persistence.",
> > -                   object_get_typename(o));
> > -        return;
> > -    }
> > -#endif
> > -
> >       fb->is_pmem = value;
> >   }
> > +#endif /* CONFIG_LIBPMEM */
> >   
> >   static void file_backend_unparent(Object *obj)
> >   {
> > @@ -180,8 +173,10 @@ file_backend_class_init(ObjectClass *oc, void *data)
> >           file_memory_backend_get_align,
> >           file_memory_backend_set_align,
> >           NULL, NULL);
> > +#ifdef CONFIG_LIBPMEM
> >       object_class_property_add_bool(oc, "pmem",
> >           file_memory_backend_get_pmem, file_memory_backend_set_pmem);
> > +#endif
> >   }
> >   
> >   static void file_backend_instance_finalize(Object *o)
> > --
> > 2.26.2