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

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 11 Dec 2018 13:29:19 +0300
Ilya Maximets <i.maximets@samsung.com> wrote:

CCing libvirt folk for an opinion

> On 10.12.2018 19:18, Igor Mammedov wrote:
> > On Tue, 27 Nov 2018 16:50:27 +0300
> > Ilya Maximets <i.maximets@samsung.com> wrote:
> > 
> > s/wihtout/without/ in subj
> >   
> >> If seals are not supported, memfd_create() will fail.
> >> Furthermore, there is no way to disable it in this case because
> >> '.seal' property is not registered.
> >>
> >> This issue leads to vhost-user-test failures on RHEL 7.2:
> >>
> >>   qemu-system-x86_64: -object memory-backend-memfd,id=mem,size=2M,: \
> >>                       failed to create memfd: Invalid argument
> >>
> >> and actually breaks the feature on such systems.
> >>
> >> Let's restrict memfd backend to systems with sealing support.
> >>
[...]
> >> @@ -177,7 +175,7 @@ static const TypeInfo memfd_backend_info = {
> >>  
> >>  static void register_types(void)
> >>  {
> >> -    if (qemu_memfd_check(0)) {
> >> +    if (qemu_memfd_check(MFD_ALLOW_SEALING)) {
> >>          type_register_static(&memfd_backend_info);  
> > that would either lead to not clear error that type doesn't exist.
> > it could be better to report sensible error from memfd_backend_memory_alloc() if
> > the feature is required but not supported by host   
> 
> I'm not sure, but this could break the libvirt capability discovering.
> 
> Current patch changes behaviour probably only for RHEL/CentOS 7.2.
> All other systems are not affected. Do you think that we need to
> change behaviour on all the systems?
you are changing behavior anyways, so when users start getting
on some of 'All other systems' start getting 'type doesn't exist'
error, they won't have a clue what's wrong. In case where we are
fixing broken defaults, shouldn't we at least do it the way that
would inform user about misconfiguration.

But I'm not insisting since memfd is fairly new, it might be fine
for device to just disappear.

[...]