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

[Michal Privoznik]

On 2/17/21 12:07 AM, John Snow wrote:
> 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.

I'm open for discussion. How do you think libvirt (or any other mgmt 
tool/user) should inspect whether given feature is available?
What libvirt currently does it issues 'qom-list-properties' with 
'typename=memory-backend-file' and inspects whether pmem attribute is 
available. Is 'qom-list' preferred?


> > 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.

That was at hand solution. If libvirt's not doing it right, I'm happy to 
make things better.

Michal