Re: [PATCH v6 4/8] KVM: Extend the memslot to support fd-based private memory

[Sean Christopherson]

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.