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

[Eduardo Habkost]

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.

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

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
> 

-- 
Eduardo