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

[Sean Christopherson <seanjc@google.com>]

On Fri, Jun 17, 2022, Sean Christopherson wrote:
> > @@ -110,6 +133,7 @@ struct kvm_userspace_memory_region {
> >   */
> >  #define KVM_MEM_LOG_DIRTY_PAGES	(1UL << 0)
> >  #define KVM_MEM_READONLY	(1UL << 1)
> > +#define KVM_MEM_PRIVATE		(1UL << 2)
> 
> Hmm, KVM_MEM_PRIVATE is technically wrong now that a "private" memslot maps private
> and/or shared memory.  Strictly speaking, we don't actually need a new flag.  Valid
> file descriptors must be >=0, so the logic for specifying a memslot that can be
> converted between private and shared could be that "(int)private_fd < 0" means
> "not convertible", i.e. derive the flag from private_fd.
> 
> And looking at the two KVM consumers of the flag, via kvm_slot_is_private(), they're
> both wrong.  Both kvm_faultin_pfn() and kvm_mmu_max_mapping_level() should operate
> on the _fault_, not the slot.  So it would actually be a positive to not have an easy
> way to query if a slot supports conversion.

I take that back, the usage in kvm_faultin_pfn() is correct, but the names ends
up being confusing because it suggests that it always faults in a private pfn.

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index b6d75016e48c..e1008f00609d 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4045,7 +4045,7 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
                        return RET_PF_EMULATE;
        }

-       if (fault->is_private) {
+       if (kvm_slot_can_be_private(slot)) {
                r = kvm_faultin_pfn_private(vcpu, fault);
                if (r != RET_PF_CONTINUE)
                        return r == RET_PF_FIXED ? RET_PF_CONTINUE : r;
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 31f704c83099..c5126190fb71 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -583,9 +583,9 @@ struct kvm_memory_slot {
        struct kvm *kvm;
 };

-static inline bool kvm_slot_is_private(const struct kvm_memory_slot *slot)
+static inline bool kvm_slot_can_be_private(const struct kvm_memory_slot *slot)
 {
-       return slot && (slot->flags & KVM_MEM_PRIVATE);
+       return slot && !!slot->private_file;
 }

 static inline bool kvm_slot_dirty_track_enabled(const struct kvm_memory_slot *slot)