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+.. SPDX-License-Identifier: GPL-2.0
+
+Design of TDP MMU for TDX support
+=================================
+This document describes a (high level) design for TDX support of KVM TDP MMU of
+x86 KVM.
+
+In this document, we use "TD" or "guest TD" to differentiate it from the current
+"VM" (Virtual Machine), which is supported by KVM today.
+
+
+Background of TDX
+=================
+TD private memory is designed to hold TD private content, encrypted by the CPU
+using the TD ephemeral key.  An encryption engine holds a table of encryption
+keys, and an encryption key is selected for each memory transaction based on a
+Host Key Identifier (HKID).  By design, the host VMM does not have access to the
+encryption keys.
+
+In the first generation of MKTME, HKID is "stolen" from the physical address by
+allocating a configurable number of bits from the top of the physical address.
+The HKID space is partitioned into shared HKIDs for legacy MKTME accesses and
+private HKIDs for SEAM-mode-only accesses.  We use 0 for the shared HKID on the
+host so that MKTME can be opaque or bypassed on the host.
+
+During TDX non-root operation (i.e. guest TD), memory accesses can be qualified
+as either shared or private, based on the value of a new SHARED bit in the Guest
+Physical Address (GPA).  The CPU translates shared GPAs using the usual VMX EPT
+(Extended Page Table) or "Shared EPT" (in this document), which resides in the
+host VMM memory.  The Shared EPT is directly managed by the host VMM - the same
+as with the current VMX.  Since guest TDs usually require I/O, and the data
+exchange needs to be done via shared memory, thus KVM needs to use the current
+EPT functionality even for TDs.
+
+The CPU translates private GPAs using a separate Secure EPT.  The Secure EPT
+pages are encrypted and integrity-protected with the TD's ephemeral private key.
+Secure EPT can be managed _indirectly_ by the host VMM, using the TDX interface
+functions (SEAMCALLs), and thus conceptually Secure EPT is a subset of EPT
+because not all functionalities are available.
+
+Since the execution of such interface functions takes much longer time than
+accessing memory directly, in KVM we use the existing TDP code to mirror the
+Secure EPT for the TD. And we think there are at least two options today in
+terms of the timing for executing such SEAMCALLs:
+
+1. synchronous, i.e. while walking the TDP page tables, or
+2. post-walk, i.e. record what needs to be done to the real Secure EPT during
+   the walk, and execute SEAMCALLs later.
+
+The option 1 seems to be more intuitive and simpler, but the Secure EPT
+concurrency rules are different from the ones of the TDP or EPT. For example,
+MEM.SEPT.RD acquire shared access to the whole Secure EPT tree of the target
+
+Secure EPT(SEPT) operations
+---------------------------
+Secure EPT is an Extended Page Table for GPA-to-HPA translation of TD private
+HPA.  A Secure EPT is designed to be encrypted with the TD's ephemeral private
+key. SEPT pages are allocated by the host VMM via Intel TDX functions, but their
+content is intended to be hidden and is not architectural.
+
+Unlike the conventional EPT, the CPU can't directly read/write its entry.
+Instead, TDX SEAMCALL API is used.  Several SEAMCALLs correspond to operation on
+the EPT entry.
+
+* TDH.MEM.SEPT.ADD():
+
+  Add a secure EPT page from the secure EPT tree.  This corresponds to updating
+  the non-leaf EPT entry with present bit set
+
+* TDH.MEM.SEPT.REMOVE():
+
+  Remove the secure page from the secure EPT tree.  There is no corresponding
+  to the EPT operation.
+
+* TDH.MEM.SEPT.RD():
+
+  Read the secure EPT entry.  This corresponds to reading the EPT entry as
+  memory.  Please note that this is much slower than direct memory reading.
+
+* TDH.MEM.PAGE.ADD() and TDH.MEM.PAGE.AUG():
+
+  Add a private page to the secure EPT tree.  This corresponds to updating the
+  leaf EPT entry with present bit set.
+
+* THD.MEM.PAGE.REMOVE():
+
+  Remove a private page from the secure EPT tree.  There is no corresponding
+  to the EPT operation.
+
+* TDH.MEM.RANGE.BLOCK():
+
+  This (mostly) corresponds to clearing the present bit of the leaf EPT entry.
+  Note that the private page is still linked in the secure EPT.  To remove it
+  from the secure EPT, TDH.MEM.SEPT.REMOVE() and TDH.MEM.PAGE.REMOVE() needs to
+  be called.
+
+* TDH.MEM.TRACK():
+
+  Increment the TLB epoch counter. This (mostly) corresponds to EPT TLB flush.
+  Note that the private page is still linked in the secure EPT.  To remove it
+  from the secure EPT, tdh_mem_page_remove() needs to be called.
+
+
+Adding private page
+-------------------
+The procedure of populating the private page looks as follows.
+
+1. TDH.MEM.SEPT.ADD(512G level)
+2. TDH.MEM.SEPT.ADD(1G level)
+3. TDH.MEM.SEPT.ADD(2M level)
+4. TDH.MEM.PAGE.AUG(4K level)
+
+Those operations correspond to updating the EPT entries.
+
+Dropping private page and TLB shootdown
+---------------------------------------
+The procedure of dropping the private page looks as follows.
+
+1. TDH.MEM.RANGE.BLOCK(4K level)
+
+   This mostly corresponds to clear the present bit in the EPT entry.  This
+   prevents (or blocks) TLB entry from creating in the future.  Note that the
+   private page is still linked in the secure EPT tree and the existing cache
+   entry in the TLB isn't flushed.
+
+2. TDH.MEM.TRACK(range) and TLB shootdown
+
+   This mostly corresponds to the EPT TLB shootdown.  Because all vcpus share
+   the same Secure EPT, all vcpus need to flush TLB.
+
+   * TDH.MEM.TRACK(range) by one vcpu.  It increments the global internal TLB
+     epoch counter.
+
+   * send IPI to remote vcpus
+   * Other vcpu exits to VMM from guest TD and then re-enter. TDH.VP.ENTER().
+   * TDH.VP.ENTER() checks the TLB epoch counter and If its TLB is old, flush
+     TLB.
+
+   Note that only single vcpu issues tdh_mem_track().
+
+   Note that the private page is still linked in the secure EPT tree, unlike the
+   conventional EPT.
+
+3. TDH.MEM.PAGE.PROMOTE, TDH.MEM.PAGEDEMOTE(), TDH.MEM.PAGE.RELOCATE(), or
+   TDH.MEM.PAGE.REMOVE()
+
+   There is no corresponding operation to the conventional EPT.
+
+   * When changing page size (e.g. 4K <-> 2M) TDH.MEM.PAGE.PROMOTE() or
+     TDH.MEM.PAGE.DEMOTE() is used.  During those operation, the guest page is
+     kept referenced in the Secure EPT.
+
+   * When migrating page, TDH.MEM.PAGE.RELOCATE().  This requires both source
+     page and destination page.
+   * when destroying TD, TDH.MEM.PAGE.REMOVE() removes the private page from the
+     secure EPT tree.  In this case TLB shootdown is not needed because vcpus
+     don't run any more.
+
+The basic idea for TDX support
+==============================
+Because shared EPT is the same as the existing EPT, use the existing logic for
+shared EPT.  On the other hand, secure EPT requires additional operations
+instead of directly reading/writing of the EPT entry.
+
+On EPT violation, The KVM mmu walks down the EPT tree from the root, determines
+the EPT entry to operate, and updates the entry. If necessary, a TLB shootdown
+is done.  Because it's very slow to directly walk secure EPT by TDX SEAMCALL,
+TDH.MEM.SEPT.RD(), the mirror of secure EPT is created and maintained.  Add
+hooks to KVM MMU to reuse the existing code.
+
+EPT violation on shared GPA
+---------------------------
+(1) EPT violation on shared GPA or zapping shared GPA
+    ::
+
+        walk down shared EPT tree (the existing code)
+                |
+                |
+                V
+        shared EPT tree (CPU refers.)
+
+(2) update the EPT entry. (the existing code)
+
+    TLB shootdown in the case of zapping.
+
+
+EPT violation on private GPA
+----------------------------
+(1) EPT violation on private GPA or zapping private GPA
+    ::
+
+        walk down the mirror of secure EPT tree (mostly same as the existing code)
+            |
+            |
+            V
+        mirror of secure EPT tree (KVM MMU software only. reuse of the existing code)
+
+(2) update the (mirrored) EPT entry. (mostly same as the existing code)
+
+(3) call the hooks with what EPT entry is changed
+    ::
+
+           |
+        NEW: hooks in KVM MMU
+           |
+           V
+        secure EPT root(CPU refers)
+
+(4) the TDX backend calls necessary TDX SEAMCALLs to update real secure EPT.
+
+The major modification is to add hooks for the TDX backend for additional
+operations and to pass down which EPT, shared EPT, or private EPT is used, and
+twist the behavior if we're operating on private EPT.
+
+The following depicts the relationship.
+::
+
+                    KVM                             |       TDX module
+                     |                              |           |
+        -------------+----------                    |           |
+        |                      |                    |           |
+        V                      V                    |           |
+     shared GPA           private GPA               |           |
+  CPU shared EPT pointer  KVM private EPT pointer   |  CPU secure EPT pointer
+        |                      |                    |           |
+        |                      |                    |           |
+        V                      V                    |           V
+  shared EPT                private EPT<-------mirror----->Secure EPT
+        |                      |                    |           |
+        |                      \--------------------+------\    |
+        |                                           |      |    |
+        V                                           |      V    V
+  shared guest page                                 |    private guest page
+                                                    |
+                                                    |
+                              non-encrypted memory  |    encrypted memory
+                                                    |
+
+shared EPT: CPU and KVM walk with shared GPA
+            Maintained by the existing code
+private EPT: KVM walks with private GPA
+             Maintained by the twisted existing code
+secure EPT: CPU walks with private GPA.
+            Maintained by TDX module with TDX SEAMCALLs via hooks
+
+
+Tracking private EPT page
+=========================
+Shared EPT pages are managed by struct kvm_mmu_page.  They are linked in a list
+structure.  When necessary, the list is traversed to operate on.  Private EPT
+pages have different characteristics.  For example, private pages can't be
+swapped out.  When shrinking memory, we'd like to traverse only shared EPT pages
+and skip private EPT pages.  Likewise, page migration isn't supported for
+private pages (yet).  Introduce an additional list to track shared EPT pages and
+track private EPT pages independently.
+
+At the beginning of EPT violation, the fault handler knows fault GPA, thus it
+knows which EPT to operate on, private or shared.  If it's private EPT,
+an additional task is done.  Something like "if (private) { callback a hook }".
+Since the fault handler has deep function calls, it's cumbersome to hold the
+information of which EPT is operating.  Options to mitigate it are
+
+1. Pass the information as an argument for the function call.
+2. Record the information in struct kvm_mmu_page somehow.
+3. Record the information in vcpu structure.
+
+Option 2 was chosen.  Because option 1 requires modifying all the functions.  It
+would affect badly to the normal case.  Option 3 doesn't work well because in
+some cases, we need to walk both private and shared EPT.
+
+The role of the EPT page can be utilized and one bit can be curved out from
+unused bits in struct kvm_mmu_page_role.  When allocating the EPT page,
+initialize the information. Mostly struct kvm_mmu_page is available because
+we're operating on EPT pages.
+
+
+The conversion of private GPA and shared GPA
+============================================
+A page of a given GPA can be assigned to only private GPA xor shared GPA at one
+time.  The GPA can't be accessed simultaneously via both private GPA and shared
+GPA.  On guest startup, all the GPAs are assigned as private.  Guest converts
+the range of GPA to shared (or private) from private (or shared) by MapGPA
+hypercall.  MapGPA hypercall takes the start GPA and the size of the region.  If
+the given start GPA is shared, VMM converts the region into shared (if it's
+already shared, nop).  If the start GPA is private, VMM converts the region into
+private.  It implies the guest won't access the unmapped region. private(or
+shared) region after converting to shared(or private).
+
+If the guest TD triggers an EPT violation on the already converted region, the
+access won't be allowed (loop in EPT violation) until other vcpu converts back
+the region.
+
+KVM MMU records which GPA is allowed to access, private or shared by xarray.
+
+
+The original TDP MMU and race condition
+=======================================
+Because vcpus share the EPT, once the EPT entry is zapped, we need to shootdown
+TLB.  Send IPI to remote vcpus.  Remote vcpus flush their down TLBs.  Until TLB
+shootdown is done, vcpus may reference the zapped guest page.
+
+TDP MMU uses read lock of mmu_lock to mitigate vcpu contention.  When read lock
+is obtained, it depends on the atomic update of the EPT entry.  (On the other
+hand legacy MMU uses write lock.)  When vcpu is populating/zapping the EPT entry
+with a read lock held, other vcpu may be populating or zapping the same EPT
+entry at the same time.
+
+To avoid the race condition, the entry is frozen.  It means the EPT entry is set
+to the special value, REMOVED_SPTE which clears the present bit.  And then after
+TLB shootdown, update the EPT entry to the final value.
+
+Concurrent zapping
+------------------
+1. read lock
+2. freeze the EPT entry (atomically set the value to REMOVED_SPTE)
+   If other vcpu froze the entry, restart page fault.
+3. TLB shootdown
+
+   * send IPI to remote vcpus
+   * TLB flush (local and remote)
+
+   For each entry update, TLB shootdown is needed because of the
+   concurrency.
+4. atomically set the EPT entry to the final value
+5. read unlock
+
+Concurrent populating
+---------------------
+In the case of populating the non-present EPT entry, atomically update the EPT
+entry.
+
+1. read lock
+
+2. atomically update the EPT entry
+   If other vcpu frozen the entry or updated the entry, restart page fault.
+
+3. read unlock
+
+In the case of updating the present EPT entry (e.g. page migration), the
+operation is split into two.  Zapping the entry and populating the entry.
+
+1. read lock
+2. zap the EPT entry.  follow the concurrent zapping case.
+3. populate the non-present EPT entry.
+4. read unlock
+
+Non-concurrent batched zapping
+------------------------------
+In some cases, zapping the ranges is done exclusively with a write lock held.
+In this case, the TLB shootdown is batched into one.
+
+1. write lock
+2. zap the EPT entries by traversing them
+3. TLB shootdown
+4. write unlock
+
+For Secure EPT, TDX SEAMCALLs are needed in addition to updating the mirrored
+EPT entry.
+
+TDX concurrent zapping
+----------------------
+Add a hook for TDX SEAMCALLs at the step of the TLB shootdown.
+
+1. read lock
+2. freeze the EPT entry(set the value to REMOVED_SPTE)
+3. TLB shootdown via a hook
+
+   * TLB.MEM.RANGE.BLOCK()
+   * TLB.MEM.TRACK()
+   * send IPI to remote vcpus
+
+4. set the EPT entry to the final value
+5. read unlock
+
+TDX concurrent populating
+-------------------------
+TDX SEAMCALLs are required in addition to operating the mirrored EPT entry.  The
+frozen entry is utilized by following the zapping case to avoid the race
+condition.  A hook can be added.
+
+1. read lock
+2. freeze the EPT entry
+3. hook
+
+   * TDH_MEM_SEPT_ADD() for non-leaf or TDH_MEM_PAGE_AUG() for leaf.
+
+4. set the EPT entry to the final value
+5. read unlock
+
+Without freezing the entry, the following race can happen.  Suppose two vcpus
+are faulting on the same GPA and the 2M and 4K level entries aren't populated
+yet.
+
+* vcpu 1: update 2M level EPT entry
+* vcpu 2: update 4K level EPT entry
+* vcpu 2: TDX SEAMCALL to update 4K secure EPT entry => error
+* vcpu 1: TDX SEAMCALL to update 2M secure EPT entry
+
+
+TDX non-concurrent batched zapping
+----------------------------------
+For simplicity, the procedure of concurrent populating is utilized.  The
+procedure can be optimized later.
+
+
+Co-existing with unmapping guest private memory
+===============================================
+TODO.  This needs to be addressed.
+
+
+Restrictions or future work
+===========================
+The following features aren't supported yet at the moment.
+
+* optimizing non-concurrent zap
+* Large page
+* Page migration
