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

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Message-ID: <CAGtprH9fDMiuk3JGSS12M-wFoqRj+sjdtEHJFS_5QfKX7aGkRQ@mail.gmail.com>
Date: Tue, 6 May 2025 06:18:55 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: pbonzini@...hat.com, seanjc@...gle.com, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, x86@...nel.org, rick.p.edgecombe@...el.com, 
	dave.hansen@...el.com, kirill.shutemov@...el.com, tabba@...gle.com, 
	ackerleytng@...gle.com, quic_eberman@...cinc.com, michael.roth@....com, 
	david@...hat.com, vbabka@...e.cz, jroedel@...e.de, thomas.lendacky@....com, 
	pgonda@...gle.com, zhiquan1.li@...el.com, fan.du@...el.com, 
	jun.miao@...el.com, ira.weiny@...el.com, isaku.yamahata@...el.com, 
	xiaoyao.li@...el.com, binbin.wu@...ux.intel.com, chao.p.peng@...el.com
Subject: Re: [RFC PATCH 08/21] KVM: TDX: Increase/decrease folio ref for huge pages

On Mon, May 5, 2025 at 11:07 PM Yan Zhao <yan.y.zhao@...el.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@...el.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@...el.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.

>
>
> > 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.

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.

[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