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 (mpriv...@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 :|