1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
|
.. SPDX-License-Identifier: GPL-2.0
=================
KVM Lock Overview
=================
1. Acquisition Orders
---------------------
The acquisition orders for mutexes are as follows:
- kvm->lock is taken outside vcpu->mutex
- kvm->lock is taken outside kvm->slots_lock and kvm->irq_lock
- kvm->slots_lock is taken outside kvm->irq_lock, though acquiring
them together is quite rare.
- kvm->mn_active_invalidate_count ensures that pairs of
invalidate_range_start() and invalidate_range_end() callbacks
use the same memslots array. kvm->slots_lock and kvm->slots_arch_lock
are taken on the waiting side in install_new_memslots, so MMU notifiers
must not take either kvm->slots_lock or kvm->slots_arch_lock.
For SRCU:
- ``synchronize_srcu(&kvm->srcu)`` is called _inside_
the kvm->slots_lock critical section, therefore kvm->slots_lock
cannot be taken inside a kvm->srcu read-side critical section.
Instead, kvm->slots_arch_lock is released before the call
to ``synchronize_srcu()`` and _can_ be taken inside a
kvm->srcu read-side critical section.
- kvm->lock is taken inside kvm->srcu, therefore
``synchronize_srcu(&kvm->srcu)`` cannot be called inside
a kvm->lock critical section. If you cannot delay the
call until after kvm->lock is released, use ``call_srcu``.
On x86:
- vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock
- kvm->arch.mmu_lock is an rwlock. kvm->arch.tdp_mmu_pages_lock and
kvm->arch.mmu_unsync_pages_lock are taken inside kvm->arch.mmu_lock, and
cannot be taken without already holding kvm->arch.mmu_lock (typically with
``read_lock`` for the TDP MMU, thus the need for additional spinlocks).
Everything else is a leaf: no other lock is taken inside the critical
sections.
2. Exception
------------
Fast page fault:
Fast page fault is the fast path which fixes the guest page fault out of
the mmu-lock on x86. Currently, the page fault can be fast in one of the
following two cases:
1. Access Tracking: The SPTE is not present, but it is marked for access
tracking. That means we need to restore the saved R/X bits. This is
described in more detail later below.
2. Write-Protection: The SPTE is present and the fault is caused by
write-protect. That means we just need to change the W bit of the spte.
What we use to avoid all the race is the Host-writable bit and MMU-writable bit
on the spte:
- Host-writable means the gfn is writable in the host kernel page tables and in
its KVM memslot.
- MMU-writable means the gfn is writable in the guest's mmu and it is not
write-protected by shadow page write-protection.
On fast page fault path, we will use cmpxchg to atomically set the spte W
bit if spte.HOST_WRITEABLE = 1 and spte.WRITE_PROTECT = 1, to restore the saved
R/X bits if for an access-traced spte, or both. This is safe because whenever
changing these bits can be detected by cmpxchg.
But we need carefully check these cases:
1) The mapping from gfn to pfn
The mapping from gfn to pfn may be changed since we can only ensure the pfn
is not changed during cmpxchg. This is a ABA problem, for example, below case
will happen:
+------------------------------------------------------------------------+
| At the beginning:: |
| |
| gpte = gfn1 |
| gfn1 is mapped to pfn1 on host |
| spte is the shadow page table entry corresponding with gpte and |
| spte = pfn1 |
+------------------------------------------------------------------------+
| On fast page fault path: |
+------------------------------------+-----------------------------------+
| CPU 0: | CPU 1: |
+------------------------------------+-----------------------------------+
| :: | |
| | |
| old_spte = *spte; | |
+------------------------------------+-----------------------------------+
| | pfn1 is swapped out:: |
| | |
| | spte = 0; |
| | |
| | pfn1 is re-alloced for gfn2. |
| | |
| | gpte is changed to point to |
| | gfn2 by the guest:: |
| | |
| | spte = pfn1; |
+------------------------------------+-----------------------------------+
| :: |
| |
| if (cmpxchg(spte, old_spte, old_spte+W) |
| mark_page_dirty(vcpu->kvm, gfn1) |
| OOPS!!! |
+------------------------------------------------------------------------+
We dirty-log for gfn1, that means gfn2 is lost in dirty-bitmap.
For direct sp, we can easily avoid it since the spte of direct sp is fixed
to gfn. For indirect sp, we disabled fast page fault for simplicity.
A solution for indirect sp could be to pin the gfn, for example via
kvm_vcpu_gfn_to_pfn_atomic, before the cmpxchg. After the pinning:
- We have held the refcount of pfn that means the pfn can not be freed and
be reused for another gfn.
- The pfn is writable and therefore it cannot be shared between different gfns
by KSM.
Then, we can ensure the dirty bitmaps is correctly set for a gfn.
2) Dirty bit tracking
In the origin code, the spte can be fast updated (non-atomically) if the
spte is read-only and the Accessed bit has already been set since the
Accessed bit and Dirty bit can not be lost.
But it is not true after fast page fault since the spte can be marked
writable between reading spte and updating spte. Like below case:
+------------------------------------------------------------------------+
| At the beginning:: |
| |
| spte.W = 0 |
| spte.Accessed = 1 |
+------------------------------------+-----------------------------------+
| CPU 0: | CPU 1: |
+------------------------------------+-----------------------------------+
| In mmu_spte_clear_track_bits():: | |
| | |
| old_spte = *spte; | |
| | |
| | |
| /* 'if' condition is satisfied. */| |
| if (old_spte.Accessed == 1 && | |
| old_spte.W == 0) | |
| spte = 0ull; | |
+------------------------------------+-----------------------------------+
| | on fast page fault path:: |
| | |
| | spte.W = 1 |
| | |
| | memory write on the spte:: |
| | |
| | spte.Dirty = 1 |
+------------------------------------+-----------------------------------+
| :: | |
| | |
| else | |
| old_spte = xchg(spte, 0ull) | |
| if (old_spte.Accessed == 1) | |
| kvm_set_pfn_accessed(spte.pfn);| |
| if (old_spte.Dirty == 1) | |
| kvm_set_pfn_dirty(spte.pfn); | |
| OOPS!!! | |
+------------------------------------+-----------------------------------+
The Dirty bit is lost in this case.
In order to avoid this kind of issue, we always treat the spte as "volatile"
if it can be updated out of mmu-lock, see spte_has_volatile_bits(), it means,
the spte is always atomically updated in this case.
3) flush tlbs due to spte updated
If the spte is updated from writable to readonly, we should flush all TLBs,
otherwise rmap_write_protect will find a read-only spte, even though the
writable spte might be cached on a CPU's TLB.
As mentioned before, the spte can be updated to writable out of mmu-lock on
fast page fault path, in order to easily audit the path, we see if TLBs need
be flushed caused by this reason in mmu_spte_update() since this is a common
function to update spte (present -> present).
Since the spte is "volatile" if it can be updated out of mmu-lock, we always
atomically update the spte, the race caused by fast page fault can be avoided,
See the comments in spte_has_volatile_bits() and mmu_spte_update().
Lockless Access Tracking:
This is used for Intel CPUs that are using EPT but do not support the EPT A/D
bits. In this case, PTEs are tagged as A/D disabled (using ignored bits), and
when the KVM MMU notifier is called to track accesses to a page (via
kvm_mmu_notifier_clear_flush_young), it marks the PTE not-present in hardware
by clearing the RWX bits in the PTE and storing the original R & X bits in more
unused/ignored bits. When the VM tries to access the page later on, a fault is
generated and the fast page fault mechanism described above is used to
atomically restore the PTE to a Present state. The W bit is not saved when the
PTE is marked for access tracking and during restoration to the Present state,
the W bit is set depending on whether or not it was a write access. If it
wasn't, then the W bit will remain clear until a write access happens, at which
time it will be set using the Dirty tracking mechanism described above.
3. Reference
------------
``kvm_lock``
^^^^^^^^^^^^
:Type: mutex
:Arch: any
:Protects: - vm_list
``kvm_count_lock``
^^^^^^^^^^^^^^^^^^
:Type: raw_spinlock_t
:Arch: any
:Protects: - hardware virtualization enable/disable
:Comment: 'raw' because hardware enabling/disabling must be atomic /wrt
migration.
``kvm->mn_invalidate_lock``
^^^^^^^^^^^^^^^^^^^^^^^^^^^
:Type: spinlock_t
:Arch: any
:Protects: mn_active_invalidate_count, mn_memslots_update_rcuwait
``kvm_arch::tsc_write_lock``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
:Type: raw_spinlock_t
:Arch: x86
:Protects: - kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}
- tsc offset in vmcb
:Comment: 'raw' because updating the tsc offsets must not be preempted.
``kvm->mmu_lock``
^^^^^^^^^^^^^^^^^
:Type: spinlock_t or rwlock_t
:Arch: any
:Protects: -shadow page/shadow tlb entry
:Comment: it is a spinlock since it is used in mmu notifier.
``kvm->srcu``
^^^^^^^^^^^^^
:Type: srcu lock
:Arch: any
:Protects: - kvm->memslots
- kvm->buses
:Comment: The srcu read lock must be held while accessing memslots (e.g.
when using gfn_to_* functions) and while accessing in-kernel
MMIO/PIO address->device structure mapping (kvm->buses).
The srcu index can be stored in kvm_vcpu->srcu_idx per vcpu
if it is needed by multiple functions.
``kvm->slots_arch_lock``
^^^^^^^^^^^^^^^^^^^^^^^^
:Type: mutex
:Arch: any (only needed on x86 though)
:Protects: any arch-specific fields of memslots that have to be modified
in a ``kvm->srcu`` read-side critical section.
:Comment: must be held before reading the pointer to the current memslots,
until after all changes to the memslots are complete
``wakeup_vcpus_on_cpu_lock``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
:Type: spinlock_t
:Arch: x86
:Protects: wakeup_vcpus_on_cpu
:Comment: This is a per-CPU lock and it is used for VT-d posted-interrupts.
When VT-d posted-interrupts is supported and the VM has assigned
devices, we put the blocked vCPU on the list blocked_vcpu_on_cpu
protected by blocked_vcpu_on_cpu_lock, when VT-d hardware issues
wakeup notification event since external interrupts from the
assigned devices happens, we will find the vCPU on the list to
wakeup.
|
