James, Sorry for a late reply. I still do have one or two pure questions, but nothing directly relevant to your series. On Thu, Jan 02, 2025 at 12:53:11PM -0500, James Houghton wrote: > So I'm not pushing for KVM Userfault to replace userfaultfd; it's not > worth the extra/duplicated complexity. And at LPC, Paolo and Sean > indicated that this direction was indeed wrong. I have another way to > make this work in mind. :) Do you still want to share it, more or less? :) > > For the gmem case, userfaultfd cannot be used, so KVM Userfault isn't > replacing it. And as of right now anyway, KVM Userfault *does* provide > a complete post-copy system for gmem. > > When gmem pages can be mapped into userspace, for post-copy to remain > functional, userspace-mapped gmem will need userfaultfd integration. > Keep in mind that even after this integration happens, userfaultfd > alone will *not* be a complete post-copy solution, as vCPU faults > won't be resolved via the userspace page tables. Do you know in context of CoCo, whether a private page can be accessed at all outside of KVM? I think I'm pretty sure now a private page can never be mapped to userspace. However, can another module like vhost-kernel access it during postcopy? My impression of that is still a yes, but then how about vhost-user? Here, the "vhost-kernel" part represents a question on whether private pages can be accessed at all outside KVM. While "vhost-user" part represents a question on whether, if the previous vhost-kernel question answers as "yes it can", such access attempt can happen in another process/task (hence, not only does it lack KVM context, but also not sharing the same task context). Thanks, -- Peter Xu
