Re: [PATCH v3 kvm/queue 11/16] KVM: Add kvm_map_gfn_range

[Sean Christopherson <seanjc@google.com>]

On Fri, Dec 31, 2021, Chao Peng wrote:
> On Fri, Dec 24, 2021 at 12:13:51PM +0800, Chao Peng wrote:
> > On Thu, Dec 23, 2021 at 06:06:19PM +0000, Sean Christopherson wrote:
> > > On Thu, Dec 23, 2021, Chao Peng wrote:
> > > > This new function establishes the mapping in KVM page tables for a
> > > > given gfn range. It can be used in the memory fallocate callback for
> > > > memfd based memory to establish the mapping for KVM secondary MMU when
> > > > the pages are allocated in the memory backend.
> > > 
> > > NAK, under no circumstance should KVM install SPTEs in response to allocating
> > > memory in a file.   The correct thing to do is to invalidate the gfn range
> > > associated with the newly mapped range, i.e. wipe out any shared SPTEs associated
> > > with the memslot.
> > 
> > Right, thanks.
> 
> BTW, I think the current fallocate() callback is just useless as long as
> we don't want to install KVM SPTEs in response to allocating memory in a
> file. The invalidation of the shared SPTEs should be notified through 
> mmu_notifier of the shared memory backend, not memfd_notifier of the
> private memory backend.

No, because the private fd is the final source of truth as to whether or not a
GPA is private, e.g. userspace may choose to not unmap the shared backing.
KVM's rule per Paolo's/this proposoal is that a GPA is private if it has a private
memslot and is present in the private backing store.  And the other core rule is
that KVM must never map both the private and shared variants of a GPA into the
guest.