On Fri, May 20, 2022, Andy Lutomirski wrote: > The alternative would be to have some kind of separate table or bitmap (part > of the memslot?) that tells KVM whether a GPA should map to the fd. > > What do you all think? My original proposal was to have expolicit shared vs. private memslots, and punch holes in KVM's memslots on conversion, but due to the way KVM (and userspace) handle memslot updates, conversions would be painfully slow. That's how we ended up with the current propsoal. But a dedicated KVM ioctl() to add/remove shared ranges would be easy to implement and wouldn't necessarily even need to interact with the memslots. It could be a consumer of memslots, e.g. if we wanted to disallow registering regions without an associated memslot, but I think we'd want to avoid even that because things will get messy during memslot updates, e.g. if dirty logging is toggled or a shared memory region is temporarily removed then we wouldn't want to destroy the tracking. I don't think we'd want to use a bitmap, e.g. for a well-behaved guest, XArray should be far more efficient. One benefit to explicitly tracking this in KVM is that it might be useful for software-only protected VMs, e.g. KVM could mark a region in the XArray as "pending" based on guest hypercalls to share/unshare memory, and then complete the transaction when userspace invokes the ioctl() to complete the share/unshare.
