Re: [Qemu-devel] [PATCH 1/4] hostmem-memfd: enable seals only if supported

[Marc-André Lureau]

Hi

On Tue, Nov 27, 2018 at 4:37 PM Ilya Maximets <address@hidden> wrote:
>
> On 27.11.2018 15:29, Marc-André Lureau wrote:
> > Hi
> >
> > On Tue, Nov 27, 2018 at 4:02 PM Ilya Maximets <address@hidden> wrote:
> >>
> >> On 27.11.2018 15:00, Marc-André Lureau wrote:
> >>> Hi
> >>> On Tue, Nov 27, 2018 at 3:56 PM Ilya Maximets <address@hidden> wrote:
> >>>>
> >>>> On 27.11.2018 14:49, Marc-André Lureau wrote:
> >>>>> Hi
> >>>>> On Tue, Nov 27, 2018 at 3:11 PM Ilya Maximets <address@hidden> wrote:
> >>>>>>
> >>>>>> 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
> >>>>>>
> >>>>>> Signed-off-by: Ilya Maximets <address@hidden>
> >>>>>
> >>>>>
> >>>>> This will change the default behaviour of memfd backend, and may thus
> >>>>> me considered a break.
> >>>>
> >>>> This will change the default behaviour only on systems without sealing
> >>>> support. But current implementation is broken there anyway and does not
> >>>> work.
> >>>>
> >>>>>
> >>>>> Instead, memfd vhost-user-test should skipped (or tuned with
> >>>>> sealed=false if unsupported)
> >>>>
> >>>> vhost-user-test is just an example here. In fact memfd could not be
> >>>> used at all on system without sealing support. And there is no way
> >>>> to disable seals.
> >>>
> >>> which system supports memfd without sealing?
> >>
> >> RHEL 7.2. kernel version 3.10.0-327.el7.x86_64
> >
> > Correct, it was backported without sealing for some reason.
> >
> > I would rather have an explicit seal=off argument on such system
> > (because sealing is expected to be available with memfd in general)
> >
>
> But '.seal' property registering is guarded by 
> 'qemu_memfd_check(MFD_ALLOW_SEALING)'.
> And you can not disable it:
>
> qemu-system-x86_64: -object memory-backend-memfd,seal=off,id=mem,size=2M,: 
> Property '.seal' not found

Right

>
> Enabling this option on system that does not support sealing will
> probably break some libvirt feature discovering or something similar.
>
> What about adding some warning to 'memfd_backend_instance_init' if
> sealing not supported before disabling it ?

What do you think of Gerd suggestion, and disable memfd backend if
sealing is not available? (the sealing property check can be removed
then).

>
> >>
> >>>
> >>>>
> >>>>>
> >>>>>> ---
> >>>>>>  backends/hostmem-memfd.c | 4 ++--
> >>>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>>>>>
> >>>>>> diff --git a/backends/hostmem-memfd.c b/backends/hostmem-memfd.c
> >>>>>> index b6836b28e5..ee39bdbde6 100644
> >>>>>> --- a/backends/hostmem-memfd.c
> >>>>>> +++ b/backends/hostmem-memfd.c
> >>>>>> @@ -129,8 +129,8 @@ memfd_backend_instance_init(Object *obj)
> >>>>>>  {
> >>>>>>      HostMemoryBackendMemfd *m = MEMORY_BACKEND_MEMFD(obj);
> >>>>>>
> >>>>>> -    /* default to sealed file */
> >>>>>> -    m->seal = true;
> >>>>>> +    /* default to sealed file if supported */
> >>>>>> +    m->seal = qemu_memfd_check(MFD_ALLOW_SEALING);
> >>>>>>  }
> >>>>>>
> >>>>>>  static void
> >>>>>> --
> >>>>>> 2.17.1
> >>>>>>
> >>>>>
> >>>>>
> >>>>
> >>>
> >>>
> >
> >
> >



-- 
Marc-André Lureau