Re: [PATCH v10 12/41] KVM: guest_memfd: Call arch make_shared callback for to-shared conversion

[Sean Christopherson]

On Thu, Aug 13, 2026, Ackerley Tng wrote:
> Sean Christopherson <seanjc@xxxxxxxxxx> writes:
> 
> > On Thu, Aug 13, 2026, Binbin Wu wrote:
> >> On 8/8/2026 5:52 AM, Ackerley Tng via B4 Relay wrote:
> >> > +#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
> >> > +static void kvm_gmem_make_shared(struct inode *inode, pgoff_t start, pgoff_t end)
> >> > +{
> >> > +	struct folio_batch fbatch;
> >> > +	pgoff_t next = start;
> >> > +	int i;
> >> > +
> >> > +	folio_batch_init(&fbatch);
> >> > +	while (filemap_get_folios(inode->i_mapping, &next, end - 1, &fbatch)) {
> >> > +		for (i = 0; i < folio_batch_count(&fbatch); ++i) {
> >> > +			struct folio *folio = fbatch.folios[i];
> >> > +			pgoff_t start_index, end_index;
> >> > +			kvm_pfn_t start_pfn;
> >> > +			kvm_pfn_t nr_pages;
> >> > +
> >> > +			start_index = max(start, folio->index);
> >> > +			end_index = min(end, folio_next_index(folio));
> >> > +			/*
> >> > +			 * end_index is either in folio or points to
> >> > +			 * the first page of the next folio. Hence,
> >> > +			 * all pages in range [start_index, end_index)
> >> > +			 * are contiguous.
> >> > +			 */
> >> > +			start_pfn = folio_file_pfn(folio, start_index);
> >> > +			nr_pages = end_index - start_index;
> >> > +
> >> > +			kvm_arch_gmem_make_shared(start_pfn, nr_pages);
> >> > +		}
> >> > +
> >> > +		folio_batch_release(&fbatch);
> >> > +		cond_resched();
> >> > +	}
> >> > +}
> >> > +#else
> >> > +static void kvm_gmem_make_shared(struct inode *inode, pgoff_t start, pgoff_t end) {}
> >> > +#endif
> >> > +
> >> >  static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start,
> >> >  				     size_t nr_pages, uint64_t attrs,
> >> >  				     pgoff_t *err_index)
> >> > @@ -599,7 +636,12 @@ static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start,
> >> >
> >> >  	filter = to_private ? KVM_FILTER_SHARED : KVM_FILTER_PRIVATE;
> >> >  	kvm_gmem_invalidate_start(inode, start, end, filter);
> >> > +
> >> > +	if (!to_private)
> 
> Should I make this condition
> 
> 	if (!to_private && kvm_x86_ops.gmem_make_shared)
> 
> instead?
> 
> Will that help?

No, because that's bleeding x86 details into common code, which defeats the purpose
of arch hooks.  That's why I think it's worth analyzing the cost: if it's in the
noise, leave it alone.  If it's meaningful, figure out a not-too-gross way to skip
the entire thing if kvm_arch_gmem_make_shared() is a glorified nop in the end.