Re: RFC: A KVM-specific alternative to UserfaultFD

[Sean Christopherson <seanjc@google.com>]

On Wed, Nov 08, 2023, Peter Xu wrote:
> On Wed, Nov 08, 2023 at 08:56:22AM -0800, David Matlack wrote:
> > Thanks for the longer explanation. Yes kvm_read_guest() eventually
> > calls __copy_from_user() which will trigger a page fault and
> > UserfaultFD will notify userspace and wait for the page to become
> > present. In the KVM-specific proposal I outlined, calling
> > kvm_read_guest() will ultimately result in a check of the VM's present
> > bitmap and KVM will nnotify userspace and wait for the page to become
> > present if it's not, before calling __copy_from_user(). So I don't
> > expect a KVM-specific solution to have any increased maintenance
> > burden for VGIC (or any other widgets).
> 
> The question is how to support modules that do not use kvm apis at all,
> like vhost.  I raised the question in my initial reply, too.
> 
> I think if vhost is going to support gmemfd, it'll need new apis so maybe
> there'll be a chance to take that into account, but I'm not 100% sure it'll
> be the same complexity, also not sure if that's the plan even for CoCo.
>
> Or is anything like vhost not considered to be supported for gmemfd at all?

vhost shouldn't require new APIs.  To support vhost, guest_memfd would first need
to support virtio for host userspace, i.e. would need to support .mmap().  At that
point, all of the uaccess and gup() stuff in vhost should work without modification.