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

[Sean Christopherson <seanjc@google.com>]

On Fri, Jul 17, 2026, Xiaoyao Li wrote:
> On 7/17/2026 5:42 AM, Ackerley Tng wrote:
> > Xiaoyao Li <xiaoyao.li@intel.com> writes:
> > @@ -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.

Agreed.  How about I add a patch to guard the call with:

	if (kvm_arch_has_private_mem(kvm) &&
	    !(GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_INIT_SHARED))
		r = kvm_gmem_make_private(kvm, slot, gfn, folio);

which is semantically correct pre-in-place conversion.  And then when in-place
conversion comes along, that will get switched to:

	if (kvm_gmem_is_private_mem(inode, index))
		r = kvm_gmem_make_private(kvm, slot, gfn, folio);

Which IMO yields a very clean and intuitive diff.

Actually, even better would be to insert a patch to provide kvm_gmem_is_private_mem()
and kvm_gmem_is_shared_mem() as part of this prep, and pull in "KVM: guest_memfd:
Only prepare folios for private pages" with a massaged shortlog+changelog.

I.e. have this at the end of this prep work:

static bool kvm_gmem_is_private_mem(struct inode *inode, pgoff_t index)
{
	return kvm_arch_has_private_mem(kvm) && 
	       !(GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_INIT_SHARED);
}

static bool kvm_gmem_is_shared_mem(struct inode *inode, pgoff_t index)
{
	return !kvm_gmem_is_private_mem(inode, index);
}

	if (!kvm_gmem_is_shared_mem(inode, vmf->pgoff))
		return VM_FAULT_SIGBUS;

	if (kvm_gmem_is_private_mem(inode, index))
		r = kvm_gmem_make_private(kvm, slot, gfn, folio);

	
And then "KVM: guest_memfd: Introduce per-gmem attributes, use to guard user mappings"
isn't changing the semantics of the callers, it's only changing the internal
plumbing for PRIVATE vs. SHARED.