Re: [Qemu-devel] [PATCH v2 1/4] hostmem-memfd: disable for systems wihtout sealing support

[Daniel P . Berrangé]

On Wed, Jan 16, 2019 at 06:46:39PM +0300, Ilya Maximets wrote:
> 
> 
> On 16.01.2019 18:30, Eduardo Habkost wrote:
> > On Wed, Dec 12, 2018 at 07:49:36AM +0100, Gerd Hoffmann wrote:
> >> On Tue, Dec 11, 2018 at 02:09:11PM +0300, Ilya Maximets wrote:
> >>> On 11.12.2018 13:53, Daniel P. Berrangé wrote:
> >>>>>
> >>>>> Let's restrict memfd backend to systems with sealing support.
> >>>>
> >>>> I don't think we need todo that - sealing is optional in the QEMU code,
> >>>> we simply have it set to the wrong default when sealing is not available.
> >>>
> >>> That was literally what I've fixed in v1:
> >>>     https://lists.nongnu.org/archive/html/qemu-devel/2018-11/msg05483.html
> >>>
> >>> but 2 people suggested me to disable memfd entirely for this case.
> >>> Do you think I need to get patch from v1 back ?
> >>>
> >>> Gerd, Marc-André, what do you think?
> >>
> >> I still think it makes sense to require sealing support.  Sealing is
> >> very useful, and there are only a few kernel versions with memfd but
> >> without sealing.  So finding such kernels in the wild will become more
> >> rare over time.  I wouldn't worry too much about them.
> > 
> > -object memory-backend-memfd,id=mem,size=2M,seal=off still
> > works on those systems, doesn't it?  What's the rationale for
> > breaking a working configuration without following the
> > deprecation policy?
> > 
> 
> See the commit message.
> '.seal' property is not registered if sealing is not supported.
> So, there is no way to disable sealing on the system that does not support it.

As I pointed out a few lines up, this is simply because QEMU has a bug
setting seal=true as the built-in default value even when it isn't
supported. 

Regards,
Daniel
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