On Thu, Aug 1, 2024 at 3:44 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > Early warning for next week's PUCK since there's actually a topic this time. > James is going to lead a discussion on KVM userfault[*](name subject to change). Thanks for attending, everyone! We seemed to arrive at the following conclusions: 1. For guest_memfd, stage 2 mapping installation will never go through GUP / virtual addresses to do the GFN --> PFN translation, including when it supports non-private memory. 2. Something like KVM Userfault is indeed necessary to handle post-copy for guest_memfd VMs, especially when guest_memfd supports non-private memory. 3. We should not hook into the overall GFN --> HVA translation, we should only be hooking the GFN --> PFN translation steps to figure out how to create stage 2 mappings. That is, KVM's own accesses to guest memory should just go through mm/userfaultfd. 4. We don't need the concept of "async userfaults" (making KVM block when attempting to access userfault memory) in KVM Userfault. So I need to think more about what exactly the API should look like for controlling if a page should exit to userspace before KVM is allowed to map it into stage 2 and if this should apply to all of guest memory or only guest_memfd. It sounds like it may most likely be something like a per-VM bitmap that describes which pages are allowed to be mapped into stage 2, applying to all memory, not just guest_memfd memory. Even though it is solving a problem for guest_memfd specifically, it is slightly cleaner to have it apply to all memory. If this per-VM bitmap applies to all memory, then we don't need to wait for guest_memfd to support non-private memory before working on a full implementation. But if not, perhaps it makes sense to wait. There will be a 30 minute session at LPC to discuss this topic more. I hope to see you there! Here are the slides[2]. Thanks! PS: I'll be away from August 9 - 25. [2]: https://docs.google.com/presentation/d/1Al9amGumF3ZPX2Wu50mQ4nkPRZZdBJitXmMH3n7j_RE/edit?usp=sharing > I Cc'd folks a few folks that I know are interested, please forward this on > as needed. > > Early warning #2, PUCK is canceled for August 14th, as I'll be traveling, though > y'all are welcome to meet without me. > > [*] https://lore.kernel.org/all/20240710234222.2333120-1-jthoughton@xxxxxxxxxx > > Time: 6am PDT > Video: https://meet.google.com/vdb-aeqo-knk > Phone: https://tel.meet/vdb-aeqo-knk?pin=3003112178656 > > Calendar: https://calendar.google.com/calendar/u/0?cid=Y182MWE1YjFmNjQ0NzM5YmY1YmVkN2U1ZWE1ZmMzNjY5Y2UzMmEyNTQ0YzVkYjFjN2M4OTE3MDJjYTUwOTBjN2Q1QGdyb3VwLmNhbGVuZGFyLmdvb2dsZS5jb20 > Drive: https://drive.google.com/drive/folders/1aTqCrvTsQI9T4qLhhLs_l986SngGlhPH?resourcekey=0-FDy0ykM3RerZedI8R-zj4A&usp=drive_link > > Future Schedule: > Augst 7th - KVM userfault > August 14th - Canceled (Sean unavailable) > August 21st - Available > August 28th - Available
