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

From: "Daniel P. Berrangé" <berrange@redhat.com>
To: Ilya Maximets <i.maximets@samsung.com>
Cc: "Eduardo Habkost" <ehabkost@redhat.com>,
	"Gerd Hoffmann" <kraxel@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	qemu-devel@nongnu.org,
	"Marc-André Lureau" <marcandre.lureau@redhat.com>,
	"Igor Mammedov" <imammedo@redhat.com>
Subject: Re: [Qemu-devel] [PATCH v2 1/4] hostmem-memfd: disable for systems wihtout sealing support
Date: Wed, 16 Jan 2019 15:56:02 +0000	[thread overview]
Message-ID: <20190116155602.GD20275@redhat.com> (raw)
In-Reply-To: <58907276-dcc3-3f5b-bb12-647fff13325e@samsung.com>

On Wed, Jan 16, 2019 at 06:54:38PM +0300, Ilya Maximets wrote:
> On 16.01.2019 18:48, Daniel P. Berrangé wrote:
> > 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. 
> 
> So, do you think I need to return to the solution from my v1:
>   https://lists.nongnu.org/archive/html/qemu-devel/2018-11/msg05483.html

That is my preference, but we don't have  universal agreement :-(

Regards,
Daniel
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