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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Jan 31, 2023 at 02:48:54PM -0500, Peter Xu wrote:
> On Thu, Jan 26, 2023 at 12:26:45PM -0500, Peter Xu wrote:
> > On Thu, Jan 26, 2023 at 03:59:33PM +0000, Daniel P. Berrangé wrote:
> > > 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).
> > 
> > Good point.. It should be the latter, so should be doable.
> > 
> > What should be the best interface for QEMU to detect the fd passing over to
> > it?  IIUC qemu_open() requires the name to be /dev/fdset/*, but there's no
> > existing cmdline that QEMU can know which fd number to fetch from fdset to
> > be used as the /dev/userfaultfd descriptor.
> > 
> > monitor_get_fd() seems more proper, where we can define an unique string so
> > Libvirt can preset the descriptor with the same string attached to it, then
> > I can opt-in monitor_get_fd() before trying to open() or doing the syscall.
> 
> Daniel/Michal, any input here from Libvirt side?
> 
> I just noticed that monitor_get_fd() is bound to a specific monitor, then
> it seems not clear which one is from Libvirt.  If to use qemu_open() and
> add-fd I think we need another QEMU cmdline to set the fd path, iiuc.
> 
> I can also leave that for later if opening /dev/userfaultfd is already
> resolving the immediate problem in containers.

I don't have any great ideas really. If we assume the /dev/userfaultfd
is accessible to QEMU we can ignore it.

With regards,
Daniel
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