Re: [PATCH] Deprecate pmem=on with non-DAX capable backend file

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 28 Apr 2021 12:29:30 -0400
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Tue, Apr 27, 2021 at 04:48:48PM -0400, Eduardo Habkost wrote:
> > On Mon, Jan 11, 2021 at 03:33:32PM -0500, Igor Mammedov wrote:  
> > > It is not safe to pretend that emulated NVDIMM supports
> > > persistence while backend actually failed to enable it
> > > and used non-persistent mapping as fall back.
> > > Instead of falling-back, QEMU should be more strict and
> > > error out with clear message that it's not supported.
> > > So if user asks for persistence (pmem=on), they should
> > > store backing file on NVDIMM.
> > > 
> > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > > Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>  
> > 
> > I'm queueing this for 6.1, after changing "since 6.0" to "since 6.1".
> > 
> > Sorry for letting it fall through the cracks.  
> 
> This caused build failures[1] and I had to apply the following
> fixup.
Thanks!

> 
> [1] https://gitlab.com/ehabkost/qemu/-/jobs/1216917482#L3444
> 
> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> ---
>  docs/system/deprecated.rst | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
> index cc8d810be1a..c55c4bceb00 100644
> --- a/docs/system/deprecated.rst
> +++ b/docs/system/deprecated.rst
> @@ -257,6 +257,7 @@ is (a) not DAX capable or (b) not on a filesystem that support direct mapping
>  of persistent memory, is not safe and may lead to data loss or corruption in case
>  of host crash.
>  Options are:
> +
>      - modify VM configuration to set ``pmem=off`` to continue using fake NVDIMM
>        (without persistence guaranties) with backing file on non DAX storage
>      - move backing file to NVDIMM storage and keep ``pmem=on``