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

[Wang, Wei W]

On Wednesday, July 26, 2023 12:04 AM,  Sean Christopherson wrote:
> On Tue, Jul 25, 2023, Wei W Wang wrote:
> > 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.
> 
> My preference would be to require @max_order.  I can kinda sorta see why a
> generic implementation (restrictedmem) would make the param optional, but
> with gmem being KVM-internal I think it makes sense to require the param.
> Even if pKVM doesn't _currently_ need/want the order of the backing
> allocation, presumably that's because hugepage support is still on the TODO
> list, not because pKVM fundamentally doesn't need to know the order of the
> backing allocation.

Another usage is live migration. The migration flow works with 4KB pages only,
and we only need to get the pfn from the given gfn. "order" doesn't seem to be
useful for this case.