Re: [PATCH RFC gmem v1 3/8] KVM: x86: Add gmem hook for initializing memory

[Sean Christopherson <seanjc@google.com>]

On Thu, Feb 08, 2024, Suzuki K Poulose wrote:
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 8c5c017ab4e9..c7f82c2f1bcf 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -2403,9 +2403,19 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
> >   #endif /* CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES */
> >   #ifdef CONFIG_KVM_PRIVATE_MEM
> > +int __kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> > +		       gfn_t gfn, kvm_pfn_t *pfn, int *max_order, bool prep);
> >   int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> >   			      gfn_t gfn, kvm_pfn_t *pfn, int *max_order);
> >   #else
> > +static inline int __kvm_gmem_get_pfn(struct kvm *kvm,
> > +				     struct kvm_memory_slot *slot, gfn_t gfn,
> > +				     kvm_pfn_t *pfn, int *max_order)
> 
> Missing "bool prep" here ?
> 
> minor nit: Do we need to export both __kvm_gmem_get_pfn and kvm_gmem_get_pfn

Minor nit on the nit: s/export/expose.  My initial reaction was "we should *never*
export any of these" :-)

> ? I don't see anyone else using the former.
> 
> We could have :
> 
> #ifdef CONFIG_KVM_PRIVATE_MEM
> int __kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> 		       gfn_t gfn, kvm_pfn_t *pfn, int *max_order, bool prep);
> #else
> static inline int __kvm_gmem_get_pfn(struct kvm *kvm,
> 				    struct kvm_memory_slot *slot, gfn_t gfn,
> 				    kvm_pfn_t *pfn, int *max_order,
> 				    bool prep)
> {
> 	KVM_BUG_ON(1, kvm);
> 	return -EIO;
> }
> #endif
> 
> static inline int kvm_gmem_get_pfn(struct kvm *kvm,
> 				 struct kvm_memory_slot *slot, gfn_t gfn,
> 				kvm_pfn_t *pfn, int *max_order)
> {
> 	return __kvm_gmem_get_pfn(kvm, slot, gfn, pfn, max_order, true);
> }

I suspect all of this will be moot.  As discussed on the PUCK call[1] and in the
SNP enabling series[2], the plan is to have guest_memfd do (or at least initiate)
the actual copying into the backing pages, e.g. to guarantee that the pages are
in the correct state, that the appropriate locks are held, etc.

[1] https://drive.google.com/drive/folders/116YTH1h9yBZmjqeJc03cV4_AhSe-VBkc?resourcekey=0-sOGeFEUi60-znJJmZBsTHQ&usp=drive_link
[2] https://lore.kernel.org/all/ZcLuGxZ-w4fPmFxd@google.com