Re: [PATCH 0/3] util/userfaultfd: Support /dev/userfaultfd

[Daniel P . Berrangé]

On Thu, Jan 26, 2023 at 10:25:05AM -0500, Peter Xu wrote:
> On Thu, Jan 26, 2023 at 02:15:11PM +0000, Dr. David Alan Gilbert wrote:
> > * Michal Prívozník (mprivozn@redhat.com) wrote:
> > > On 1/25/23 23:40, Peter Xu wrote:
> > > > The new /dev/userfaultfd handle is superior to the system call with a
> > > > better permission control and also works for a restricted seccomp
> > > > environment.
> > > > 
> > > > The new device was only introduced in v6.1 so we need a header update.
> > > > 
> > > > Please have a look, thanks.
> > > 
> > > I was wondering whether it would make sense/be possible for mgmt app
> > > (libvirt) to pass FD for /dev/userfaultfd instead of QEMU opening it
> > > itself. But looking into the code, libvirt would need to do that when
> > > spawning QEMU because that's when QEMU itself initializes internal state
> > > and queries userfaultfd caps.
> > 
> > You also have to be careful about what the userfaultfd semantics are; I
> > can't remember them - but if you open it in one process and pass it to
> > another process, which processes address space are you trying to
> > monitor?
> 
> Yes it's a problem.  The kernel always fetches the current mm_struct* which
> represents the current context of virtual address space when creating the
> uffd handle (for either the syscall or the ioctl() approach).

At what point does the process address space get associated ? When
the /dev/userfaultfd is opened, or only when ioctl(USERFAULTFD_IOC_NEW)
is called ?  If it is the former, then we have no choice, QEMU must open
it. if it is the latter, then libvirt can open /dev/userfaultfd, pass
it to QEMU which can then do the ioctl(USERFAULTFD_IOC_NEW).

With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|