RE: [RFC PATCH v11 12/29] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

[Wang, Wei W]

On Wednesday, July 19, 2023 7:45 AM, Sean Christopherson wrote:
> +int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> +		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order) {
> +	pgoff_t index = gfn - slot->base_gfn + slot->gmem.pgoff;
> +	struct kvm_gmem *gmem;
> +	struct folio *folio;
> +	struct page *page;
> +	struct file *file;
> +
> +	file = kvm_gmem_get_file(slot);
> +	if (!file)
> +		return -EFAULT;
> +
> +	gmem = file->private_data;
> +
> +	if (WARN_ON_ONCE(xa_load(&gmem->bindings, index) != slot)) {
> +		fput(file);
> +		return -EIO;
> +	}
> +
> +	folio = kvm_gmem_get_folio(file_inode(file), index);
> +	if (!folio) {
> +		fput(file);
> +		return -ENOMEM;
> +	}
> +
> +	page = folio_file_page(folio, index);
> +
> +	*pfn = page_to_pfn(page);
> +	*max_order = compound_order(compound_head(page));

Maybe better to check if caller provided a buffer to get the max_order:
if (max_order)
	*max_order = compound_order(compound_head(page));

This is what the previous version did (restrictedmem_get_page),
so that callers who only want to get a pfn don't need to define
an unused "order" param.