Re: [RFC PATCH 08/21] KVM: TDX: Increase/decrease folio ref for huge pages

[Vishal Annapurve <vannapurve@google.com>]

On Wed, May 7, 2025 at 12:39 AM Yan Zhao <yan.y.zhao@intel.com> wrote:
>
> On Tue, May 06, 2025 at 06:18:55AM -0700, Vishal Annapurve wrote:
> > On Mon, May 5, 2025 at 11:07 PM Yan Zhao <yan.y.zhao@intel.com> wrote:
> > >
> > > On Mon, May 05, 2025 at 10:08:24PM -0700, Vishal Annapurve wrote:
> > > > On Mon, May 5, 2025 at 5:56 PM Yan Zhao <yan.y.zhao@intel.com> wrote:
> > > > >
> > > > > Sorry for the late reply, I was on leave last week.
> > > > >
> > > > > On Tue, Apr 29, 2025 at 06:46:59AM -0700, Vishal Annapurve wrote:
> > > > > > On Mon, Apr 28, 2025 at 5:52 PM Yan Zhao <yan.y.zhao@intel.com> wrote:
> > > > > > > So, we plan to remove folio_ref_add()/folio_put_refs() in future, only invoking
> > > > > > > folio_ref_add() in the event of a removal failure.
> > > > > >
> > > > > > In my opinion, the above scheme can be deployed with this series
> > > > > > itself. guest_memfd will not take away memory from TDX VMs without an
> > > > > I initially intended to add a separate patch at the end of this series to
> > > > > implement invoking folio_ref_add() only upon a removal failure. However, I
> > > > > decided against it since it's not a must before guest_memfd supports in-place
> > > > > conversion.
> > > > >
> > > > > We can include it in the next version If you think it's better.
> > > >
> > > > Ackerley is planning to send out a series for 1G Hugetlb support with
> > > > guest memfd soon, hopefully this week. Plus I don't see any reason to
> > > > hold extra refcounts in TDX stack so it would be good to clean up this
> > > > logic.
> > > >
> > > > >
> > > > > > invalidation. folio_ref_add() will not work for memory not backed by
> > > > > > page structs, but that problem can be solved in future possibly by
> > > > > With current TDX code, all memory must be backed by a page struct.
> > > > > Both tdh_mem_page_add() and tdh_mem_page_aug() require a "struct page *" rather
> > > > > than a pfn.
> > > > >
> > > > > > notifying guest_memfd of certain ranges being in use even after
> > > > > > invalidation completes.
> > > > > A curious question:
> > > > > To support memory not backed by page structs in future, is there any counterpart
> > > > > to the page struct to hold ref count and map count?
> > > > >
> > > >
> > > > I imagine the needed support will match similar semantics as VM_PFNMAP
> > > > [1] memory. No need to maintain refcounts/map counts for such physical
> > > > memory ranges as all users will be notified when mappings are
> > > > changed/removed.
> > > So, it's possible to map such memory in both shared and private EPT
> > > simultaneously?
> >
> > No, guest_memfd will still ensure that userspace can only fault in
> > shared memory regions in order to support CoCo VM usecases.
> Before guest_memfd converts a PFN from shared to private, how does it ensure
> there are no shared mappings? e.g., in [1], it uses the folio reference count
> to ensure that.
>
> Or do you believe that by eliminating the struct page, there would be no
> GUP, thereby ensuring no shared mappings by requiring all mappers to unmap in
> response to a guest_memfd invalidation notification?

Yes.

>
> As in Documentation/core-api/pin_user_pages.rst, long-term pinning users have
> no need to register mmu notifier. So why users like VFIO must register
> guest_memfd invalidation notification?

VM_PFNMAP'd memory can't be long term pinned, so users of such memory
ranges will have to adopt mechanisms to get notified. I think it would
be easy to pursue new users of guest_memfd to follow this scheme.
Irrespective of whether VM_PFNMAP'd support lands, guest_memfd
hugepage support already needs the stance of: "Guest memfd owns all
long-term refcounts on private memory" as discussed at LPC [1].

[1] https://lpc.events/event/18/contributions/1764/attachments/1409/3182/LPC%202024_%201G%20page%20support%20for%20guest_memfd.pdf
(slide 12)

>
> Besides, how would guest_memfd handle potential unmap failures? e.g. what
> happens to prevent converting a private PFN to shared if there are errors when
> TDX unmaps a private PFN or if a device refuses to stop DMAing to a PFN.

Users will have to signal such failures via the invalidation callback
results or other appropriate mechanisms. guest_memfd can relay the
failures up the call chain to the userspace.

>
> Currently, guest_memfd can rely on page ref count to avoid re-assigning a PFN
> that fails to be unmapped.
>
>
> [1] https://lore.kernel.org/all/20250328153133.3504118-5-tabba@google.com/
>
>
> > >
> > >
> > > > Any guest_memfd range updates will result in invalidations/updates of
> > > > userspace, guest, IOMMU or any other page tables referring to
> > > > guest_memfd backed pfns. This story will become clearer once the
> > > > support for PFN range allocator for backing guest_memfd starts getting
> > > > discussed.
> > > Ok. It is indeed unclear right now to support such kind of memory.
> > >
> > > Up to now, we don't anticipate TDX will allow any mapping of VM_PFNMAP memory
> > > into private EPT until TDX connect.
> >
> > There is a plan to use VM_PFNMAP memory for all of guest_memfd
> > shared/private ranges orthogonal to TDX connect usecase. With TDX
> > connect/Sev TIO, major difference would be that guest_memfd private
> > ranges will be mapped into IOMMU page tables.
> >
> > Irrespective of whether/when VM_PFNMAP memory support lands, there
> > have been discussions on not using page structs for private memory
> > ranges altogether [1] even with hugetlb allocator, which will simplify
> > seamless merge/split story for private hugepages to support memory
> > conversion. So I think the general direction we should head towards is
> > not relying on refcounts for guest_memfd private ranges and/or page
> > structs altogether.
> It's fine to use PFN, but I wonder if there're counterparts of struct page to
> keep all necessary info.
>

Story will become clearer once VM_PFNMAP'd memory support starts
getting discussed. In case of guest_memfd, there is flexibility to
store metadata for physical ranges within guest_memfd just like
shareability tracking.

>
> > I think the series [2] to work better with PFNMAP'd physical memory in
> > KVM is in the very right direction of not assuming page struct backed
> > memory ranges for guest_memfd as well.
> Note: Currently, VM_PFNMAP is usually used together with flag VM_IO. in KVM
> hva_to_pfn_remapped() only applies to "vma->vm_flags & (VM_IO | VM_PFNMAP)".
>
>
> > [1] https://lore.kernel.org/all/CAGtprH8akKUF=8+RkX_QMjp35C0bU1zxGi4v1Zm5AWCw=8V8AQ@mail.gmail.com/
> > [2] https://lore.kernel.org/linux-arm-kernel/20241010182427.1434605-1-seanjc@google.com/
> >
> > > And even in that scenario, the memory is only for private MMIO, so the backend
> > > driver is VFIO pci driver rather than guest_memfd.
> >
> > Not necessary. As I mentioned above guest_memfd ranges will be backed
> > by VM_PFNMAP memory.
> >
> > >
> > >
> > > > [1] https://elixir.bootlin.com/linux/v6.14.5/source/mm/memory.c#L6543