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

[Chao Peng]

On Fri, Jun 17, 2022 at 09:27:25PM +0000, Sean Christopherson wrote:
> 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.

Make sense, will change the naming, thanks.

> 
> 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)