KVM: TDX: Drop superfluous page pinning in S-EPT management

Don't explicitly pin pages when mapping pages into the S-EPT, guest_memfd
doesn't support page migration in any capacity, i.e. there are no migrate
callbacks because guest_memfd pages *can't* be migrated.  See the WARN in
kvm_gmem_migrate_folio().

Eliminating TDX's explicit pinning will also enable guest_memfd to support
in-place conversion between shared and private memory[1][2].  Because KVM
cannot distinguish between speculative/transient refcounts and the
intentional refcount for TDX on private pages[3], failing to release
private page refcount in TDX could cause guest_memfd to indefinitely wait
on decreasing the refcount for the splitting.

Under normal conditions, not holding an extra page refcount in TDX is safe
because guest_memfd ensures pages are retained until its invalidation
notification to KVM MMU is completed. However, if there're bugs in KVM/TDX
module, not holding an extra refcount when a page is mapped in S-EPT could
result in a page being released from guest_memfd while still mapped in the
S-EPT.  But, doing work to make a fatal error slightly less fatal is a net
negative when that extra work adds complexity and confusion.

Several approaches were considered to address the refcount issue, including
  - Attempting to modify the KVM unmap operation to return a failure,
    which was deemed too complex and potentially incorrect[4].
  - Increasing the folio reference count only upon S-EPT zapping failure[5].
  - Use page flags or page_ext to indicate a page is still used by TDX[6],
    which does not work for HVO (HugeTLB Vmemmap Optimization).
  - Setting HWPOISON bit or leveraging folio_set_hugetlb_hwpoison()[7].

Due to the complexity or inappropriateness of these approaches, and the
fact that S-EPT zapping failure is currently only possible when there are
bugs in the KVM or TDX module, which is very rare in a production kernel,
a straightforward approach of simply not holding the page reference count
in TDX was chosen[8].

When S-EPT zapping errors occur, KVM_BUG_ON() is invoked to kick off all
vCPUs and mark the VM as dead. Although there is a potential window that a
private page mapped in the S-EPT could be reallocated and used outside the
VM, the loud warning from KVM_BUG_ON() should provide sufficient debug
information. To be robust against bugs, the user can enable panic_on_warn
as normal.

Link: https://lore.kernel.org/all/cover.1747264138.git.ackerleytng@google.com [1]
Link: https://youtu.be/UnBKahkAon4 [2]
Link: https://lore.kernel.org/all/CAGtprH_ypohFy9TOJ8Emm_roT4XbQUtLKZNFcM6Fr+fhTFkE0Q@mail.gmail.com [3]
Link: https://lore.kernel.org/all/aEEEJbTzlncbRaRA@yzhao56-desk.sh.intel.com [4]
Link: https://lore.kernel.org/all/aE%2Fq9VKkmaCcuwpU@yzhao56-desk.sh.intel.com [5]
Link: https://lore.kernel.org/all/aFkeBtuNBN1RrDAJ@yzhao56-desk.sh.intel.com [6]
Link: https://lore.kernel.org/all/diqzy0tikran.fsf@ackerleytng-ctop.c.googlers.com [7]
Link: https://lore.kernel.org/all/53ea5239f8ef9d8df9af593647243c10435fd219.camel@intel.com [8]
Suggested-by: Vishal Annapurve <vannapurve@google.com>
Suggested-by: Ackerley Tng <ackerleytng@google.com>
Suggested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
[sean: extract out of hugepage series, massage changelog accordingly]
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Kai Huang <kai.huang@intel.com>
Link: https://patch.msgid.link/20251030200951.3402865-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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