Re: [PATCH v5 7/7] KVM: guest_memfd: Rework PREPARE config and hook into a more generic CONVERT

[Xiaoyao Li]

On 7/17/2026 5:42 AM, Ackerley Tng wrote:
> Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
>
> > On 7/15/2026 7:10 AM, Sean Christopherson wrote:
> > > Rework guest_memfd's "prepare" hook into a more generic "convert" flow in
> > > anticipation of supporting in-place conversion, at which point KVM will use
> > > the hook for both to-private and to-shared conversions, not just to
> > > "prepare" PRIVATE memory.
> > >
> > > Opportunistically rename kvm_gmem_prepare_folio() to kvm_gmem_make_private()
> > > to better reflect its role.
> >
> > [...]
> >
> > > @@ -90,8 +90,8 @@ static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
> > >    	gfn = ALIGN_DOWN(gfn, nr_pages);
> > >    	index = kvm_gmem_get_index(slot, gfn);
> > >
> > > -	return kvm_arch_gmem_prepare(kvm, gfn, folio_file_pfn(folio, index),
> > > -				     nr_pages, folio_order(folio));
> > > +	return kvm_arch_gmem_convert(kvm, gfn, folio_file_pfn(folio, index),
> > > +				     nr_pages, folio_order(folio), true);
> > >    #else
> > >    	return 0;
> > >    #endif
> > > @@ -798,7 +798,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> > >    		folio_mark_uptodate(folio);
> > >    	}
> > >
> > > -	r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);
> > > +	r = kvm_gmem_make_private(kvm, slot, gfn, folio);
> >
> > I think this and above renaming don't make sense, because
> > kvm_gmem_get_pfn() can be invoked for shared memory for non-Coco VMs
> > when KVM_MEMSLOT_GMEM_ONLY is set.
>
> Makes sense.
>
> > Maybe this patch can be moved to gmem in-place series and after [1]?
> >
> > [1]
> > https://lore.kernel.org/all/3b64e897-93a8-4f9c-88a9-f416ff44b09d@xxxxxxxxx/
>
> How about doing everything else in this patch in this patch, other than
> this:
>
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 173a7ed008d17..9df6927dd8f55 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -66,8 +66,8 @@ static pgoff_t kvm_gmem_get_index(struct
> kvm_memory_slot *slot, gfn_t gfn)
>    * On successful return the guest sees a zero page so as to avoid
>    * leaking host data and the up-to-date flag is set.
>    */
> -static int kvm_gmem_make_private(struct kvm *kvm, struct kvm_memory_slot *slot,
> -				 gfn_t gfn, struct folio *folio)
> +static int kvm_gmem_prepare_folio(struct kvm *kvm, struct
> kvm_memory_slot *slot,
> +				  gfn_t gfn, struct folio *folio)
>   {
>   #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
>   	unsigned long nr_pages = folio_nr_pages(folio);
> @@ -798,7 +798,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct
> kvm_memory_slot *slot,
>   		folio_mark_uptodate(folio);
>   	}
>
> -	r = kvm_gmem_make_private(kvm, slot, gfn, folio);
> +	r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);
>
>   	folio_unlock(folio);
>
> I'll move just this renaming to [1] like you suggested.
>
> I think it's okay to continue to always call prepare_folio(), and within
> the prepare_folio() function, only do conversion when the CONVERT CONFIG
> is defined.

I don't think so.

This patch not only renames kvm_arch_gmem_prepare() to 
kvm_arch_gmem_convert(), but also adds one more parameter

  'bool to_private'

and hardcodes the new parameter to true. This mean the arch callback 
will convert the folio to private unconditionally in kvm_prepare_folio() 
when CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT is enabled.

Note the CONFIG is not a per-VM thing but a build time thing. The 
unconditionally-converting-to-private semantic can also be applied to 
gmem-only memslot for non-Coco VMs whenever the 
HAVE_KVM_ARCH_GMEM_CONVERT is enabled when building the kernel.

Though the code won't do anything for gmem-only memslot for non-Coco 
VMs, the literal semantic of the function and parameter is wrong.