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

[Peter Xu]

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.

Thanks,

-- 
Peter Xu