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 and kvm_usage_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
27cpus_read_lock() vs kvm_lock:
28- Taking cpus_read_lock() outside of kvm_lock is problematic, despite that
29  being the official ordering, as it is quite easy to unknowingly trigger
30  cpus_read_lock() while holding kvm_lock.  Use caution when walking vm_list,
31  e.g. avoid complex operations when possible.
32
33For SRCU:
34
35- ``synchronize_srcu(&kvm->srcu)`` is called inside critical sections
36  for kvm->lock, vcpu->mutex and kvm->slots_lock.  These locks _cannot_
37  be taken inside a kvm->srcu read-side critical section; that is, the
38  following is broken::
39
40      srcu_read_lock(&kvm->srcu);
41      mutex_lock(&kvm->slots_lock);
42
43- kvm->slots_arch_lock instead is released before the call to
44  ``synchronize_srcu()``.  It _can_ therefore be taken inside a
45  kvm->srcu read-side critical section, for example while processing
46  a vmexit.
47
48On x86:
49
50- vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock and kvm->arch.xen.xen_lock
51
52- kvm->arch.mmu_lock is an rwlock.  kvm->arch.tdp_mmu_pages_lock and
53  kvm->arch.mmu_unsync_pages_lock are taken inside kvm->arch.mmu_lock, and
54  cannot be taken without already holding kvm->arch.mmu_lock (typically with
55  ``read_lock`` for the TDP MMU, thus the need for additional spinlocks).
56
57Everything else is a leaf: no other lock is taken inside the critical
58sections.
59
602. Exception
61------------
62
63Fast page fault:
64
65Fast page fault is the fast path which fixes the guest page fault out of
66the mmu-lock on x86. Currently, the page fault can be fast in one of the
67following two cases:
68
691. Access Tracking: The SPTE is not present, but it is marked for access
70   tracking. That means we need to restore the saved R/X bits. This is
71   described in more detail later below.
72
732. Write-Protection: The SPTE is present and the fault is caused by
74   write-protect. That means we just need to change the W bit of the spte.
75
76What we use to avoid all the races is the Host-writable bit and MMU-writable bit
77on the spte:
78
79- Host-writable means the gfn is writable in the host kernel page tables and in
80  its KVM memslot.
81- MMU-writable means the gfn is writable in the guest's mmu and it is not
82  write-protected by shadow page write-protection.
83
84On fast page fault path, we will use cmpxchg to atomically set the spte W
85bit if spte.HOST_WRITEABLE = 1 and spte.WRITE_PROTECT = 1, to restore the saved
86R/X bits if for an access-traced spte, or both. This is safe because whenever
87changing these bits can be detected by cmpxchg.
88
89But we need carefully check these cases:
90
911) The mapping from gfn to pfn
92
93The mapping from gfn to pfn may be changed since we can only ensure the pfn
94is not changed during cmpxchg. This is a ABA problem, for example, below case
95will happen:
96
97+------------------------------------------------------------------------+
98| At the beginning::                                                     |
99|                                                                        |
100|	gpte = gfn1                                                      |
101|	gfn1 is mapped to pfn1 on host                                   |
102|	spte is the shadow page table entry corresponding with gpte and  |
103|	spte = pfn1                                                      |
104+------------------------------------------------------------------------+
105| On fast page fault path:                                               |
106+------------------------------------+-----------------------------------+
107| CPU 0:                             | CPU 1:                            |
108+------------------------------------+-----------------------------------+
109| ::                                 |                                   |
110|                                    |                                   |
111|   old_spte = *spte;                |                                   |
112+------------------------------------+-----------------------------------+
113|                                    | pfn1 is swapped out::             |
114|                                    |                                   |
115|                                    |    spte = 0;                      |
116|                                    |                                   |
117|                                    | pfn1 is re-alloced for gfn2.      |
118|                                    |                                   |
119|                                    | gpte is changed to point to       |
120|                                    | gfn2 by the guest::               |
121|                                    |                                   |
122|                                    |    spte = pfn1;                   |
123+------------------------------------+-----------------------------------+
124| ::                                                                     |
125|                                                                        |
126|   if (cmpxchg(spte, old_spte, old_spte+W)                              |
127|	mark_page_dirty(vcpu->kvm, gfn1)                                 |
128|            OOPS!!!                                                     |
129+------------------------------------------------------------------------+
130
131We dirty-log for gfn1, that means gfn2 is lost in dirty-bitmap.
132
133For direct sp, we can easily avoid it since the spte of direct sp is fixed
134to gfn.  For indirect sp, we disabled fast page fault for simplicity.
135
136A solution for indirect sp could be to pin the gfn, for example via
137kvm_vcpu_gfn_to_pfn_atomic, before the cmpxchg.  After the pinning:
138
139- We have held the refcount of pfn; that means the pfn can not be freed and
140  be reused for another gfn.
141- The pfn is writable and therefore it cannot be shared between different gfns
142  by KSM.
143
144Then, we can ensure the dirty bitmaps is correctly set for a gfn.
145
1462) Dirty bit tracking
147
148In the origin code, the spte can be fast updated (non-atomically) if the
149spte is read-only and the Accessed bit has already been set since the
150Accessed bit and Dirty bit can not be lost.
151
152But it is not true after fast page fault since the spte can be marked
153writable between reading spte and updating spte. Like below case:
154
155+------------------------------------------------------------------------+
156| At the beginning::                                                     |
157|                                                                        |
158|	spte.W = 0                                                       |
159|	spte.Accessed = 1                                                |
160+------------------------------------+-----------------------------------+
161| CPU 0:                             | CPU 1:                            |
162+------------------------------------+-----------------------------------+
163| In mmu_spte_clear_track_bits()::   |                                   |
164|                                    |                                   |
165|  old_spte = *spte;                 |                                   |
166|                                    |                                   |
167|                                    |                                   |
168|  /* 'if' condition is satisfied. */|                                   |
169|  if (old_spte.Accessed == 1 &&     |                                   |
170|       old_spte.W == 0)             |                                   |
171|     spte = 0ull;                   |                                   |
172+------------------------------------+-----------------------------------+
173|                                    | on fast page fault path::         |
174|                                    |                                   |
175|                                    |    spte.W = 1                     |
176|                                    |                                   |
177|                                    | memory write on the spte::        |
178|                                    |                                   |
179|                                    |    spte.Dirty = 1                 |
180+------------------------------------+-----------------------------------+
181|  ::                                |                                   |
182|                                    |                                   |
183|   else                             |                                   |
184|     old_spte = xchg(spte, 0ull)    |                                   |
185|   if (old_spte.Accessed == 1)      |                                   |
186|     kvm_set_pfn_accessed(spte.pfn);|                                   |
187|   if (old_spte.Dirty == 1)         |                                   |
188|     kvm_set_pfn_dirty(spte.pfn);   |                                   |
189|     OOPS!!!                        |                                   |
190+------------------------------------+-----------------------------------+
191
192The Dirty bit is lost in this case.
193
194In order to avoid this kind of issue, we always treat the spte as "volatile"
195if it can be updated out of mmu-lock [see spte_has_volatile_bits()]; it means
196the spte is always atomically updated in this case.
197
1983) flush tlbs due to spte updated
199
200If the spte is updated from writable to read-only, we should flush all TLBs,
201otherwise rmap_write_protect will find a read-only spte, even though the
202writable spte might be cached on a CPU's TLB.
203
204As mentioned before, the spte can be updated to writable out of mmu-lock on
205fast page fault path. In order to easily audit the path, we see if TLBs needing
206to be flushed caused this reason in mmu_spte_update() since this is a common
207function to update spte (present -> present).
208
209Since the spte is "volatile" if it can be updated out of mmu-lock, we always
210atomically update the spte and the race caused by fast page fault can be avoided.
211See the comments in spte_has_volatile_bits() and mmu_spte_update().
212
213Lockless Access Tracking:
214
215This is used for Intel CPUs that are using EPT but do not support the EPT A/D
216bits. In this case, PTEs are tagged as A/D disabled (using ignored bits), and
217when the KVM MMU notifier is called to track accesses to a page (via
218kvm_mmu_notifier_clear_flush_young), it marks the PTE not-present in hardware
219by clearing the RWX bits in the PTE and storing the original R & X bits in more
220unused/ignored bits. When the VM tries to access the page later on, a fault is
221generated and the fast page fault mechanism described above is used to
222atomically restore the PTE to a Present state. The W bit is not saved when the
223PTE is marked for access tracking and during restoration to the Present state,
224the W bit is set depending on whether or not it was a write access. If it
225wasn't, then the W bit will remain clear until a write access happens, at which
226time it will be set using the Dirty tracking mechanism described above.
227
2283. Reference
229------------
230
231``kvm_lock``
232^^^^^^^^^^^^
233
234:Type:		mutex
235:Arch:		any
236:Protects:	- vm_list
237
238``kvm_usage_lock``
239^^^^^^^^^^^^^^^^^^
240
241:Type:		mutex
242:Arch:		any
243:Protects:	- kvm_usage_count
244		- hardware virtualization enable/disable
245:Comment:	Exists because using kvm_lock leads to deadlock (see earlier comment
246		on cpus_read_lock() vs kvm_lock).  Note, KVM also disables CPU hotplug via
247		cpus_read_lock() when enabling/disabling virtualization.
248
249``kvm->mn_invalidate_lock``
250^^^^^^^^^^^^^^^^^^^^^^^^^^^
251
252:Type:          spinlock_t
253:Arch:          any
254:Protects:      mn_active_invalidate_count, mn_memslots_update_rcuwait
255
256``kvm_arch::tsc_write_lock``
257^^^^^^^^^^^^^^^^^^^^^^^^^^^^
258
259:Type:		raw_spinlock_t
260:Arch:		x86
261:Protects:	- kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}
262		- tsc offset in vmcb
263:Comment:	'raw' because updating the tsc offsets must not be preempted.
264
265``kvm->mmu_lock``
266^^^^^^^^^^^^^^^^^
267:Type:		spinlock_t or rwlock_t
268:Arch:		any
269:Protects:	-shadow page/shadow tlb entry
270:Comment:	it is a spinlock since it is used in mmu notifier.
271
272``kvm->srcu``
273^^^^^^^^^^^^^
274:Type:		srcu lock
275:Arch:		any
276:Protects:	- kvm->memslots
277		- kvm->buses
278:Comment:	The srcu read lock must be held while accessing memslots (e.g.
279		when using gfn_to_* functions) and while accessing in-kernel
280		MMIO/PIO address->device structure mapping (kvm->buses).
281		The srcu index can be stored in kvm_vcpu->srcu_idx per vcpu
282		if it is needed by multiple functions.
283
284``kvm->slots_arch_lock``
285^^^^^^^^^^^^^^^^^^^^^^^^
286:Type:          mutex
287:Arch:          any (only needed on x86 though)
288:Protects:      any arch-specific fields of memslots that have to be modified
289                in a ``kvm->srcu`` read-side critical section.
290:Comment:       must be held before reading the pointer to the current memslots,
291                until after all changes to the memslots are complete
292
293``wakeup_vcpus_on_cpu_lock``
294^^^^^^^^^^^^^^^^^^^^^^^^^^^^
295:Type:		spinlock_t
296:Arch:		x86
297:Protects:	wakeup_vcpus_on_cpu
298:Comment:	This is a per-CPU lock and it is used for VT-d posted-interrupts.
299		When VT-d posted-interrupts are supported and the VM has assigned
300		devices, we put the blocked vCPU on the list blocked_vcpu_on_cpu
301		protected by blocked_vcpu_on_cpu_lock. When VT-d hardware issues
302		wakeup notification event since external interrupts from the
303		assigned devices happens, we will find the vCPU on the list to
304		wakeup.
305
306``vendor_module_lock``
307^^^^^^^^^^^^^^^^^^^^^^
308:Type:		mutex
309:Arch:		x86
310:Protects:	loading a vendor module (kvm_amd or kvm_intel)
311:Comment:	Exists because using kvm_lock leads to deadlock.  kvm_lock is taken
312    in notifiers, e.g. __kvmclock_cpufreq_notifier(), that may be invoked while
313    cpu_hotplug_lock is held, e.g. from cpufreq_boost_trigger_state(), and many
314    operations need to take cpu_hotplug_lock when loading a vendor module, e.g.
315    updating static calls.
316