1.. SPDX-License-Identifier: GPL-2.0
2
3=================
4KVM Lock Overview
5=================
6
71. Acquisition Orders
8---------------------
9
10The acquisition orders for mutexes are as follows:
11
12- cpus_read_lock() is taken outside kvm_lock
13
14- kvm->lock is taken outside vcpu->mutex
15
16- kvm->lock is taken outside kvm->slots_lock and kvm->irq_lock
17
18- kvm->slots_lock is taken outside kvm->irq_lock, though acquiring
19  them together is quite rare.
20
21- kvm->mn_active_invalidate_count ensures that pairs of
22  invalidate_range_start() and invalidate_range_end() callbacks
23  use the same memslots array.  kvm->slots_lock and kvm->slots_arch_lock
24  are taken on the waiting side when modifying memslots, so MMU notifiers
25  must not take either kvm->slots_lock or kvm->slots_arch_lock.
26
27For SRCU:
28
29- ``synchronize_srcu(&kvm->srcu)`` is called inside critical sections
30  for kvm->lock, vcpu->mutex and kvm->slots_lock.  These locks _cannot_
31  be taken inside a kvm->srcu read-side critical section; that is, the
32  following is broken::
33
34      srcu_read_lock(&kvm->srcu);
35      mutex_lock(&kvm->slots_lock);
36
37- kvm->slots_arch_lock instead is released before the call to
38  ``synchronize_srcu()``.  It _can_ therefore be taken inside a
39  kvm->srcu read-side critical section, for example while processing
40  a vmexit.
41
42On x86:
43
44- vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock and kvm->arch.xen.xen_lock
45
46- kvm->arch.mmu_lock is an rwlock.  kvm->arch.tdp_mmu_pages_lock and
47  kvm->arch.mmu_unsync_pages_lock are taken inside kvm->arch.mmu_lock, and
48  cannot be taken without already holding kvm->arch.mmu_lock (typically with
49  ``read_lock`` for the TDP MMU, thus the need for additional spinlocks).
50
51Everything else is a leaf: no other lock is taken inside the critical
52sections.
53
542. Exception
55------------
56
57Fast page fault:
58
59Fast page fault is the fast path which fixes the guest page fault out of
60the mmu-lock on x86. Currently, the page fault can be fast in one of the
61following two cases:
62
631. Access Tracking: The SPTE is not present, but it is marked for access
64   tracking. That means we need to restore the saved R/X bits. This is
65   described in more detail later below.
66
672. Write-Protection: The SPTE is present and the fault is caused by
68   write-protect. That means we just need to change the W bit of the spte.
69
70What we use to avoid all the races is the Host-writable bit and MMU-writable bit
71on the spte:
72
73- Host-writable means the gfn is writable in the host kernel page tables and in
74  its KVM memslot.
75- MMU-writable means the gfn is writable in the guest's mmu and it is not
76  write-protected by shadow page write-protection.
77
78On fast page fault path, we will use cmpxchg to atomically set the spte W
79bit if spte.HOST_WRITEABLE = 1 and spte.WRITE_PROTECT = 1, to restore the saved
80R/X bits if for an access-traced spte, or both. This is safe because whenever
81changing these bits can be detected by cmpxchg.
82
83But we need carefully check these cases:
84
851) The mapping from gfn to pfn
86
87The mapping from gfn to pfn may be changed since we can only ensure the pfn
88is not changed during cmpxchg. This is a ABA problem, for example, below case
89will happen:
90
91+------------------------------------------------------------------------+
92| At the beginning::                                                     |
93|                                                                        |
94|	gpte = gfn1                                                      |
95|	gfn1 is mapped to pfn1 on host                                   |
96|	spte is the shadow page table entry corresponding with gpte and  |
97|	spte = pfn1                                                      |
98+------------------------------------------------------------------------+
99| On fast page fault path:                                               |
100+------------------------------------+-----------------------------------+
101| CPU 0:                             | CPU 1:                            |
102+------------------------------------+-----------------------------------+
103| ::                                 |                                   |
104|                                    |                                   |
105|   old_spte = *spte;                |                                   |
106+------------------------------------+-----------------------------------+
107|                                    | pfn1 is swapped out::             |
108|                                    |                                   |
109|                                    |    spte = 0;                      |
110|                                    |                                   |
111|                                    | pfn1 is re-alloced for gfn2.      |
112|                                    |                                   |
113|                                    | gpte is changed to point to       |
114|                                    | gfn2 by the guest::               |
115|                                    |                                   |
116|                                    |    spte = pfn1;                   |
117+------------------------------------+-----------------------------------+
118| ::                                                                     |
119|                                                                        |
120|   if (cmpxchg(spte, old_spte, old_spte+W)                              |
121|	mark_page_dirty(vcpu->kvm, gfn1)                                 |
122|            OOPS!!!                                                     |
123+------------------------------------------------------------------------+
124
125We dirty-log for gfn1, that means gfn2 is lost in dirty-bitmap.
126
127For direct sp, we can easily avoid it since the spte of direct sp is fixed
128to gfn.  For indirect sp, we disabled fast page fault for simplicity.
129
130A solution for indirect sp could be to pin the gfn, for example via
131kvm_vcpu_gfn_to_pfn_atomic, before the cmpxchg.  After the pinning:
132
133- We have held the refcount of pfn; that means the pfn can not be freed and
134  be reused for another gfn.
135- The pfn is writable and therefore it cannot be shared between different gfns
136  by KSM.
137
138Then, we can ensure the dirty bitmaps is correctly set for a gfn.
139
1402) Dirty bit tracking
141
142In the origin code, the spte can be fast updated (non-atomically) if the
143spte is read-only and the Accessed bit has already been set since the
144Accessed bit and Dirty bit can not be lost.
145
146But it is not true after fast page fault since the spte can be marked
147writable between reading spte and updating spte. Like below case:
148
149+------------------------------------------------------------------------+
150| At the beginning::                                                     |
151|                                                                        |
152|	spte.W = 0                                                       |
153|	spte.Accessed = 1                                                |
154+------------------------------------+-----------------------------------+
155| CPU 0:                             | CPU 1:                            |
156+------------------------------------+-----------------------------------+
157| In mmu_spte_clear_track_bits()::   |                                   |
158|                                    |                                   |
159|  old_spte = *spte;                 |                                   |
160|                                    |                                   |
161|                                    |                                   |
162|  /* 'if' condition is satisfied. */|                                   |
163|  if (old_spte.Accessed == 1 &&     |                                   |
164|       old_spte.W == 0)             |                                   |
165|     spte = 0ull;                   |                                   |
166+------------------------------------+-----------------------------------+
167|                                    | on fast page fault path::         |
168|                                    |                                   |
169|                                    |    spte.W = 1                     |
170|                                    |                                   |
171|                                    | memory write on the spte::        |
172|                                    |                                   |
173|                                    |    spte.Dirty = 1                 |
174+------------------------------------+-----------------------------------+
175|  ::                                |                                   |
176|                                    |                                   |
177|   else                             |                                   |
178|     old_spte = xchg(spte, 0ull)    |                                   |
179|   if (old_spte.Accessed == 1)      |                                   |
180|     kvm_set_pfn_accessed(spte.pfn);|                                   |
181|   if (old_spte.Dirty == 1)         |                                   |
182|     kvm_set_pfn_dirty(spte.pfn);   |                                   |
183|     OOPS!!!                        |                                   |
184+------------------------------------+-----------------------------------+
185
186The Dirty bit is lost in this case.
187
188In order to avoid this kind of issue, we always treat the spte as "volatile"
189if it can be updated out of mmu-lock [see spte_has_volatile_bits()]; it means
190the spte is always atomically updated in this case.
191
1923) flush tlbs due to spte updated
193
194If the spte is updated from writable to read-only, we should flush all TLBs,
195otherwise rmap_write_protect will find a read-only spte, even though the
196writable spte might be cached on a CPU's TLB.
197
198As mentioned before, the spte can be updated to writable out of mmu-lock on
199fast page fault path. In order to easily audit the path, we see if TLBs needing
200to be flushed caused this reason in mmu_spte_update() since this is a common
201function to update spte (present -> present).
202
203Since the spte is "volatile" if it can be updated out of mmu-lock, we always
204atomically update the spte and the race caused by fast page fault can be avoided.
205See the comments in spte_has_volatile_bits() and mmu_spte_update().
206
207Lockless Access Tracking:
208
209This is used for Intel CPUs that are using EPT but do not support the EPT A/D
210bits. In this case, PTEs are tagged as A/D disabled (using ignored bits), and
211when the KVM MMU notifier is called to track accesses to a page (via
212kvm_mmu_notifier_clear_flush_young), it marks the PTE not-present in hardware
213by clearing the RWX bits in the PTE and storing the original R & X bits in more
214unused/ignored bits. When the VM tries to access the page later on, a fault is
215generated and the fast page fault mechanism described above is used to
216atomically restore the PTE to a Present state. The W bit is not saved when the
217PTE is marked for access tracking and during restoration to the Present state,
218the W bit is set depending on whether or not it was a write access. If it
219wasn't, then the W bit will remain clear until a write access happens, at which
220time it will be set using the Dirty tracking mechanism described above.
221
2223. Reference
223------------
224
225``kvm_lock``
226^^^^^^^^^^^^
227
228:Type:		mutex
229:Arch:		any
230:Protects:	- vm_list
231		- kvm_usage_count
232		- hardware virtualization enable/disable
233:Comment:	KVM also disables CPU hotplug via cpus_read_lock() during
234		enable/disable.
235
236``kvm->mn_invalidate_lock``
237^^^^^^^^^^^^^^^^^^^^^^^^^^^
238
239:Type:          spinlock_t
240:Arch:          any
241:Protects:      mn_active_invalidate_count, mn_memslots_update_rcuwait
242
243``kvm_arch::tsc_write_lock``
244^^^^^^^^^^^^^^^^^^^^^^^^^^^^
245
246:Type:		raw_spinlock_t
247:Arch:		x86
248:Protects:	- kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}
249		- tsc offset in vmcb
250:Comment:	'raw' because updating the tsc offsets must not be preempted.
251
252``kvm->mmu_lock``
253^^^^^^^^^^^^^^^^^
254:Type:		spinlock_t or rwlock_t
255:Arch:		any
256:Protects:	-shadow page/shadow tlb entry
257:Comment:	it is a spinlock since it is used in mmu notifier.
258
259``kvm->srcu``
260^^^^^^^^^^^^^
261:Type:		srcu lock
262:Arch:		any
263:Protects:	- kvm->memslots
264		- kvm->buses
265:Comment:	The srcu read lock must be held while accessing memslots (e.g.
266		when using gfn_to_* functions) and while accessing in-kernel
267		MMIO/PIO address->device structure mapping (kvm->buses).
268		The srcu index can be stored in kvm_vcpu->srcu_idx per vcpu
269		if it is needed by multiple functions.
270
271``kvm->slots_arch_lock``
272^^^^^^^^^^^^^^^^^^^^^^^^
273:Type:          mutex
274:Arch:          any (only needed on x86 though)
275:Protects:      any arch-specific fields of memslots that have to be modified
276                in a ``kvm->srcu`` read-side critical section.
277:Comment:       must be held before reading the pointer to the current memslots,
278                until after all changes to the memslots are complete
279
280``wakeup_vcpus_on_cpu_lock``
281^^^^^^^^^^^^^^^^^^^^^^^^^^^^
282:Type:		spinlock_t
283:Arch:		x86
284:Protects:	wakeup_vcpus_on_cpu
285:Comment:	This is a per-CPU lock and it is used for VT-d posted-interrupts.
286		When VT-d posted-interrupts are supported and the VM has assigned
287		devices, we put the blocked vCPU on the list blocked_vcpu_on_cpu
288		protected by blocked_vcpu_on_cpu_lock. When VT-d hardware issues
289		wakeup notification event since external interrupts from the
290		assigned devices happens, we will find the vCPU on the list to
291		wakeup.
292
293``vendor_module_lock``
294^^^^^^^^^^^^^^^^^^^^^^^^^^^^
295:Type:		mutex
296:Arch:		x86
297:Protects:	loading a vendor module (kvm_amd or kvm_intel)
298:Comment:	Exists because using kvm_lock leads to deadlock.  cpu_hotplug_lock is
299    taken outside of kvm_lock, e.g. in KVM's CPU online/offline callbacks, and
300    many operations need to take cpu_hotplug_lock when loading a vendor module,
301    e.g. updating static calls.
302