1 .. SPDX-License-Identifier: GPL-2.0 <<
2 <<
3 ================= 1 =================
4 KVM VCPU Requests 2 KVM VCPU Requests
5 ================= 3 =================
6 4
7 Overview 5 Overview
8 ======== 6 ========
9 7
10 KVM supports an internal API enabling threads 8 KVM supports an internal API enabling threads to request a VCPU thread to
11 perform some activity. For example, a thread 9 perform some activity. For example, a thread may request a VCPU to flush
12 its TLB with a VCPU request. The API consists 10 its TLB with a VCPU request. The API consists of the following functions::
13 11
14 /* Check if any requests are pending for VCP 12 /* Check if any requests are pending for VCPU @vcpu. */
15 bool kvm_request_pending(struct kvm_vcpu *vc 13 bool kvm_request_pending(struct kvm_vcpu *vcpu);
16 14
17 /* Check if VCPU @vcpu has request @req pend 15 /* Check if VCPU @vcpu has request @req pending. */
18 bool kvm_test_request(int req, struct kvm_vc 16 bool kvm_test_request(int req, struct kvm_vcpu *vcpu);
19 17
20 /* Clear request @req for VCPU @vcpu. */ 18 /* Clear request @req for VCPU @vcpu. */
21 void kvm_clear_request(int req, struct kvm_v 19 void kvm_clear_request(int req, struct kvm_vcpu *vcpu);
22 20
23 /* 21 /*
24 * Check if VCPU @vcpu has request @req pend 22 * Check if VCPU @vcpu has request @req pending. When the request is
25 * pending it will be cleared and a memory b 23 * pending it will be cleared and a memory barrier, which pairs with
26 * another in kvm_make_request(), will be is 24 * another in kvm_make_request(), will be issued.
27 */ 25 */
28 bool kvm_check_request(int req, struct kvm_v 26 bool kvm_check_request(int req, struct kvm_vcpu *vcpu);
29 27
30 /* 28 /*
31 * Make request @req of VCPU @vcpu. Issues a 29 * Make request @req of VCPU @vcpu. Issues a memory barrier, which pairs
32 * with another in kvm_check_request(), prio 30 * with another in kvm_check_request(), prior to setting the request.
33 */ 31 */
34 void kvm_make_request(int req, struct kvm_vc 32 void kvm_make_request(int req, struct kvm_vcpu *vcpu);
35 33
36 /* Make request @req of all VCPUs of the VM 34 /* Make request @req of all VCPUs of the VM with struct kvm @kvm. */
37 bool kvm_make_all_cpus_request(struct kvm *k 35 bool kvm_make_all_cpus_request(struct kvm *kvm, unsigned int req);
38 36
39 Typically a requester wants the VCPU to perfor 37 Typically a requester wants the VCPU to perform the activity as soon
40 as possible after making the request. This me 38 as possible after making the request. This means most requests
41 (kvm_make_request() calls) are followed by a c 39 (kvm_make_request() calls) are followed by a call to kvm_vcpu_kick(),
42 and kvm_make_all_cpus_request() has the kickin 40 and kvm_make_all_cpus_request() has the kicking of all VCPUs built
43 into it. 41 into it.
44 42
45 VCPU Kicks 43 VCPU Kicks
46 ---------- 44 ----------
47 45
48 The goal of a VCPU kick is to bring a VCPU thr 46 The goal of a VCPU kick is to bring a VCPU thread out of guest mode in
49 order to perform some KVM maintenance. To do 47 order to perform some KVM maintenance. To do so, an IPI is sent, forcing
50 a guest mode exit. However, a VCPU thread may 48 a guest mode exit. However, a VCPU thread may not be in guest mode at the
51 time of the kick. Therefore, depending on the 49 time of the kick. Therefore, depending on the mode and state of the VCPU
52 thread, there are two other actions a kick may 50 thread, there are two other actions a kick may take. All three actions
53 are listed below: 51 are listed below:
54 52
55 1) Send an IPI. This forces a guest mode exit 53 1) Send an IPI. This forces a guest mode exit.
56 2) Waking a sleeping VCPU. Sleeping VCPUs are 54 2) Waking a sleeping VCPU. Sleeping VCPUs are VCPU threads outside guest
57 mode that wait on waitqueues. Waking them 55 mode that wait on waitqueues. Waking them removes the threads from
58 the waitqueues, allowing the threads to run 56 the waitqueues, allowing the threads to run again. This behavior
59 may be suppressed, see KVM_REQUEST_NO_WAKEU 57 may be suppressed, see KVM_REQUEST_NO_WAKEUP below.
60 3) Nothing. When the VCPU is not in guest mod 58 3) Nothing. When the VCPU is not in guest mode and the VCPU thread is not
61 sleeping, then there is nothing to do. 59 sleeping, then there is nothing to do.
62 60
63 VCPU Mode 61 VCPU Mode
64 --------- 62 ---------
65 63
66 VCPUs have a mode state, ``vcpu->mode``, that 64 VCPUs have a mode state, ``vcpu->mode``, that is used to track whether the
67 guest is running in guest mode or not, as well 65 guest is running in guest mode or not, as well as some specific
68 outside guest mode states. The architecture m 66 outside guest mode states. The architecture may use ``vcpu->mode`` to
69 ensure VCPU requests are seen by VCPUs (see "E 67 ensure VCPU requests are seen by VCPUs (see "Ensuring Requests Are Seen"),
70 as well as to avoid sending unnecessary IPIs ( 68 as well as to avoid sending unnecessary IPIs (see "IPI Reduction"), and
71 even to ensure IPI acknowledgements are waited 69 even to ensure IPI acknowledgements are waited upon (see "Waiting for
72 Acknowledgements"). The following modes are d 70 Acknowledgements"). The following modes are defined:
73 71
74 OUTSIDE_GUEST_MODE 72 OUTSIDE_GUEST_MODE
75 73
76 The VCPU thread is outside guest mode. 74 The VCPU thread is outside guest mode.
77 75
78 IN_GUEST_MODE 76 IN_GUEST_MODE
79 77
80 The VCPU thread is in guest mode. 78 The VCPU thread is in guest mode.
81 79
82 EXITING_GUEST_MODE 80 EXITING_GUEST_MODE
83 81
84 The VCPU thread is transitioning from IN_GUE 82 The VCPU thread is transitioning from IN_GUEST_MODE to
85 OUTSIDE_GUEST_MODE. 83 OUTSIDE_GUEST_MODE.
86 84
87 READING_SHADOW_PAGE_TABLES 85 READING_SHADOW_PAGE_TABLES
88 86
89 The VCPU thread is outside guest mode, but i 87 The VCPU thread is outside guest mode, but it wants the sender of
90 certain VCPU requests, namely KVM_REQ_TLB_FL 88 certain VCPU requests, namely KVM_REQ_TLB_FLUSH, to wait until the VCPU
91 thread is done reading the page tables. 89 thread is done reading the page tables.
92 90
93 VCPU Request Internals 91 VCPU Request Internals
94 ====================== 92 ======================
95 93
96 VCPU requests are simply bit indices of the `` 94 VCPU requests are simply bit indices of the ``vcpu->requests`` bitmap.
97 This means general bitops, like those document 95 This means general bitops, like those documented in [atomic-ops]_ could
98 also be used, e.g. :: 96 also be used, e.g. ::
99 97
100 clear_bit(KVM_REQ_UNBLOCK & KVM_REQUEST_MASK !! 98 clear_bit(KVM_REQ_UNHALT & KVM_REQUEST_MASK, &vcpu->requests);
101 99
102 However, VCPU request users should refrain fro 100 However, VCPU request users should refrain from doing so, as it would
103 break the abstraction. The first 8 bits are r 101 break the abstraction. The first 8 bits are reserved for architecture
104 independent requests; all additional bits are !! 102 independent requests, all additional bits are available for architecture
105 dependent requests. 103 dependent requests.
106 104
107 Architecture Independent Requests 105 Architecture Independent Requests
108 --------------------------------- 106 ---------------------------------
109 107
110 KVM_REQ_TLB_FLUSH 108 KVM_REQ_TLB_FLUSH
111 109
112 KVM's common MMU notifier may need to flush 110 KVM's common MMU notifier may need to flush all of a guest's TLB
113 entries, calling kvm_flush_remote_tlbs() to 111 entries, calling kvm_flush_remote_tlbs() to do so. Architectures that
114 choose to use the common kvm_flush_remote_tl 112 choose to use the common kvm_flush_remote_tlbs() implementation will
115 need to handle this VCPU request. 113 need to handle this VCPU request.
116 114
117 KVM_REQ_VM_DEAD !! 115 KVM_REQ_MMU_RELOAD
118 116
119 This request informs all VCPUs that the VM i !! 117 When shadow page tables are used and memory slots are removed it's
120 fatal error or because the VM's state has be !! 118 necessary to inform each VCPU to completely refresh the tables. This
>> 119 request is used for that.
121 120
122 KVM_REQ_UNBLOCK 121 KVM_REQ_UNBLOCK
123 122
124 This request informs the vCPU to exit kvm_vc 123 This request informs the vCPU to exit kvm_vcpu_block. It is used for
125 example from timer handlers that run on the 124 example from timer handlers that run on the host on behalf of a vCPU,
126 or in order to update the interrupt routing 125 or in order to update the interrupt routing and ensure that assigned
127 devices will wake up the vCPU. 126 devices will wake up the vCPU.
128 127
129 KVM_REQ_OUTSIDE_GUEST_MODE !! 128 KVM_REQ_UNHALT
130 129
131 This "request" ensures the target vCPU has e !! 130 This request may be made from the KVM common function kvm_vcpu_block(),
132 sender of the request continuing on. No act !! 131 which is used to emulate an instruction that causes a CPU to halt until
133 and so no request is actually logged for the !! 132 one of an architectural specific set of events and/or interrupts is
134 to a "kick", but unlike a kick it guarantees !! 133 received (determined by checking kvm_arch_vcpu_runnable()). When that
135 guest mode. A kick only guarantees the vCPU !! 134 event or interrupt arrives kvm_vcpu_block() makes the request. This is
136 future, e.g. a previous kick may have starte !! 135 in contrast to when kvm_vcpu_block() returns due to any other reason,
137 guarantee the to-be-kicked vCPU has fully ex !! 136 such as a pending signal, which does not indicate the VCPU's halt
>> 137 emulation should stop, and therefore does not make the request.
138 138
139 KVM_REQUEST_MASK 139 KVM_REQUEST_MASK
140 ---------------- 140 ----------------
141 141
142 VCPU requests should be masked by KVM_REQUEST_ 142 VCPU requests should be masked by KVM_REQUEST_MASK before using them with
143 bitops. This is because only the lower 8 bits 143 bitops. This is because only the lower 8 bits are used to represent the
144 request's number. The upper bits are used as 144 request's number. The upper bits are used as flags. Currently only two
145 flags are defined. 145 flags are defined.
146 146
147 VCPU Request Flags 147 VCPU Request Flags
148 ------------------ 148 ------------------
149 149
150 KVM_REQUEST_NO_WAKEUP 150 KVM_REQUEST_NO_WAKEUP
151 151
152 This flag is applied to requests that only n 152 This flag is applied to requests that only need immediate attention
153 from VCPUs running in guest mode. That is, 153 from VCPUs running in guest mode. That is, sleeping VCPUs do not need
154 to be awakened for these requests. Sleeping !! 154 to be awaken for these requests. Sleeping VCPUs will handle the
155 requests when they are awakened later for so !! 155 requests when they are awaken later for some other reason.
156 156
157 KVM_REQUEST_WAIT 157 KVM_REQUEST_WAIT
158 158
159 When requests with this flag are made with k 159 When requests with this flag are made with kvm_make_all_cpus_request(),
160 then the caller will wait for each VCPU to a 160 then the caller will wait for each VCPU to acknowledge its IPI before
161 proceeding. This flag only applies to VCPUs 161 proceeding. This flag only applies to VCPUs that would receive IPIs.
162 If, for example, the VCPU is sleeping, so no 162 If, for example, the VCPU is sleeping, so no IPI is necessary, then
163 the requesting thread does not wait. This m 163 the requesting thread does not wait. This means that this flag may be
164 safely combined with KVM_REQUEST_NO_WAKEUP. 164 safely combined with KVM_REQUEST_NO_WAKEUP. See "Waiting for
165 Acknowledgements" for more information about 165 Acknowledgements" for more information about requests with
166 KVM_REQUEST_WAIT. 166 KVM_REQUEST_WAIT.
167 167
168 VCPU Requests with Associated State 168 VCPU Requests with Associated State
169 =================================== 169 ===================================
170 170
171 Requesters that want the receiving VCPU to han 171 Requesters that want the receiving VCPU to handle new state need to ensure
172 the newly written state is observable to the r 172 the newly written state is observable to the receiving VCPU thread's CPU
173 by the time it observes the request. This mea 173 by the time it observes the request. This means a write memory barrier
174 must be inserted after writing the new state a 174 must be inserted after writing the new state and before setting the VCPU
175 request bit. Additionally, on the receiving V 175 request bit. Additionally, on the receiving VCPU thread's side, a
176 corresponding read barrier must be inserted af 176 corresponding read barrier must be inserted after reading the request bit
177 and before proceeding to read the new state as 177 and before proceeding to read the new state associated with it. See
178 scenario 3, Message and Flag, of [lwn-mb]_ and 178 scenario 3, Message and Flag, of [lwn-mb]_ and the kernel documentation
179 [memory-barriers]_. 179 [memory-barriers]_.
180 180
181 The pair of functions, kvm_check_request() and 181 The pair of functions, kvm_check_request() and kvm_make_request(), provide
182 the memory barriers, allowing this requirement 182 the memory barriers, allowing this requirement to be handled internally by
183 the API. 183 the API.
184 184
185 Ensuring Requests Are Seen 185 Ensuring Requests Are Seen
186 ========================== 186 ==========================
187 187
188 When making requests to VCPUs, we want to avoi 188 When making requests to VCPUs, we want to avoid the receiving VCPU
189 executing in guest mode for an arbitrary long 189 executing in guest mode for an arbitrary long time without handling the
190 request. We can be sure this won't happen as 190 request. We can be sure this won't happen as long as we ensure the VCPU
191 thread checks kvm_request_pending() before ent 191 thread checks kvm_request_pending() before entering guest mode and that a
192 kick will send an IPI to force an exit from gu 192 kick will send an IPI to force an exit from guest mode when necessary.
193 Extra care must be taken to cover the period a 193 Extra care must be taken to cover the period after the VCPU thread's last
194 kvm_request_pending() check and before it has 194 kvm_request_pending() check and before it has entered guest mode, as kick
195 IPIs will only trigger guest mode exits for VC 195 IPIs will only trigger guest mode exits for VCPU threads that are in guest
196 mode or at least have already disabled interru 196 mode or at least have already disabled interrupts in order to prepare to
197 enter guest mode. This means that an optimize 197 enter guest mode. This means that an optimized implementation (see "IPI
198 Reduction") must be certain when it's safe to 198 Reduction") must be certain when it's safe to not send the IPI. One
199 solution, which all architectures except s390 199 solution, which all architectures except s390 apply, is to:
200 200
201 - set ``vcpu->mode`` to IN_GUEST_MODE between 201 - set ``vcpu->mode`` to IN_GUEST_MODE between disabling the interrupts and
202 the last kvm_request_pending() check; 202 the last kvm_request_pending() check;
203 - enable interrupts atomically when entering t 203 - enable interrupts atomically when entering the guest.
204 204
205 This solution also requires memory barriers to 205 This solution also requires memory barriers to be placed carefully in both
206 the requesting thread and the receiving VCPU. 206 the requesting thread and the receiving VCPU. With the memory barriers we
207 can exclude the possibility of a VCPU thread o 207 can exclude the possibility of a VCPU thread observing
208 !kvm_request_pending() on its last check and t 208 !kvm_request_pending() on its last check and then not receiving an IPI for
209 the next request made of it, even if the reque 209 the next request made of it, even if the request is made immediately after
210 the check. This is done by way of the Dekker 210 the check. This is done by way of the Dekker memory barrier pattern
211 (scenario 10 of [lwn-mb]_). As the Dekker pat 211 (scenario 10 of [lwn-mb]_). As the Dekker pattern requires two variables,
212 this solution pairs ``vcpu->mode`` with ``vcpu 212 this solution pairs ``vcpu->mode`` with ``vcpu->requests``. Substituting
213 them into the pattern gives:: 213 them into the pattern gives::
214 214
215 CPU1 CPU2 215 CPU1 CPU2
216 ================= ==== 216 ================= =================
217 local_irq_disable(); 217 local_irq_disable();
218 WRITE_ONCE(vcpu->mode, IN_GUEST_MODE); kvm_ 218 WRITE_ONCE(vcpu->mode, IN_GUEST_MODE); kvm_make_request(REQ, vcpu);
219 smp_mb(); smp_ 219 smp_mb(); smp_mb();
220 if (kvm_request_pending(vcpu)) { if ( 220 if (kvm_request_pending(vcpu)) { if (READ_ONCE(vcpu->mode) ==
221 221 IN_GUEST_MODE) {
222 ...abort guest entry... 222 ...abort guest entry... ...send IPI...
223 } } 223 } }
224 224
225 As stated above, the IPI is only useful for VC 225 As stated above, the IPI is only useful for VCPU threads in guest mode or
226 that have already disabled interrupts. This i 226 that have already disabled interrupts. This is why this specific case of
227 the Dekker pattern has been extended to disabl 227 the Dekker pattern has been extended to disable interrupts before setting
228 ``vcpu->mode`` to IN_GUEST_MODE. WRITE_ONCE() 228 ``vcpu->mode`` to IN_GUEST_MODE. WRITE_ONCE() and READ_ONCE() are used to
229 pedantically implement the memory barrier patt 229 pedantically implement the memory barrier pattern, guaranteeing the
230 compiler doesn't interfere with ``vcpu->mode`` 230 compiler doesn't interfere with ``vcpu->mode``'s carefully planned
231 accesses. 231 accesses.
232 232
233 IPI Reduction 233 IPI Reduction
234 ------------- 234 -------------
235 235
236 As only one IPI is needed to get a VCPU to che 236 As only one IPI is needed to get a VCPU to check for any/all requests,
237 then they may be coalesced. This is easily do 237 then they may be coalesced. This is easily done by having the first IPI
238 sending kick also change the VCPU mode to some 238 sending kick also change the VCPU mode to something !IN_GUEST_MODE. The
239 transitional state, EXITING_GUEST_MODE, is use 239 transitional state, EXITING_GUEST_MODE, is used for this purpose.
240 240
241 Waiting for Acknowledgements 241 Waiting for Acknowledgements
242 ---------------------------- 242 ----------------------------
243 243
244 Some requests, those with the KVM_REQUEST_WAIT 244 Some requests, those with the KVM_REQUEST_WAIT flag set, require IPIs to
245 be sent, and the acknowledgements to be waited 245 be sent, and the acknowledgements to be waited upon, even when the target
246 VCPU threads are in modes other than IN_GUEST_ 246 VCPU threads are in modes other than IN_GUEST_MODE. For example, one case
247 is when a target VCPU thread is in READING_SHA 247 is when a target VCPU thread is in READING_SHADOW_PAGE_TABLES mode, which
248 is set after disabling interrupts. To support 248 is set after disabling interrupts. To support these cases, the
249 KVM_REQUEST_WAIT flag changes the condition fo 249 KVM_REQUEST_WAIT flag changes the condition for sending an IPI from
250 checking that the VCPU is IN_GUEST_MODE to che 250 checking that the VCPU is IN_GUEST_MODE to checking that it is not
251 OUTSIDE_GUEST_MODE. 251 OUTSIDE_GUEST_MODE.
252 252
253 Request-less VCPU Kicks 253 Request-less VCPU Kicks
254 ----------------------- 254 -----------------------
255 255
256 As the determination of whether or not to send 256 As the determination of whether or not to send an IPI depends on the
257 two-variable Dekker memory barrier pattern, th 257 two-variable Dekker memory barrier pattern, then it's clear that
258 request-less VCPU kicks are almost never corre 258 request-less VCPU kicks are almost never correct. Without the assurance
259 that a non-IPI generating kick will still resu 259 that a non-IPI generating kick will still result in an action by the
260 receiving VCPU, as the final kvm_request_pendi 260 receiving VCPU, as the final kvm_request_pending() check does for
261 request-accompanying kicks, then the kick may 261 request-accompanying kicks, then the kick may not do anything useful at
262 all. If, for instance, a request-less kick wa 262 all. If, for instance, a request-less kick was made to a VCPU that was
263 just about to set its mode to IN_GUEST_MODE, m 263 just about to set its mode to IN_GUEST_MODE, meaning no IPI is sent, then
264 the VCPU thread may continue its entry without 264 the VCPU thread may continue its entry without actually having done
265 whatever it was the kick was meant to initiate 265 whatever it was the kick was meant to initiate.
266 266
267 One exception is x86's posted interrupt mechan 267 One exception is x86's posted interrupt mechanism. In this case, however,
268 even the request-less VCPU kick is coupled wit 268 even the request-less VCPU kick is coupled with the same
269 local_irq_disable() + smp_mb() pattern describ 269 local_irq_disable() + smp_mb() pattern described above; the ON bit
270 (Outstanding Notification) in the posted inter 270 (Outstanding Notification) in the posted interrupt descriptor takes the
271 role of ``vcpu->requests``. When sending a po 271 role of ``vcpu->requests``. When sending a posted interrupt, PIR.ON is
272 set before reading ``vcpu->mode``; dually, in 272 set before reading ``vcpu->mode``; dually, in the VCPU thread,
273 vmx_sync_pir_to_irr() reads PIR after setting 273 vmx_sync_pir_to_irr() reads PIR after setting ``vcpu->mode`` to
274 IN_GUEST_MODE. 274 IN_GUEST_MODE.
275 275
276 Additional Considerations 276 Additional Considerations
277 ========================= 277 =========================
278 278
279 Sleeping VCPUs 279 Sleeping VCPUs
280 -------------- 280 --------------
281 281
282 VCPU threads may need to consider requests bef 282 VCPU threads may need to consider requests before and/or after calling
283 functions that may put them to sleep, e.g. kvm 283 functions that may put them to sleep, e.g. kvm_vcpu_block(). Whether they
284 do or not, and, if they do, which requests nee 284 do or not, and, if they do, which requests need consideration, is
285 architecture dependent. kvm_vcpu_block() call 285 architecture dependent. kvm_vcpu_block() calls kvm_arch_vcpu_runnable()
286 to check if it should awaken. One reason to d 286 to check if it should awaken. One reason to do so is to provide
287 architectures a function where requests may be 287 architectures a function where requests may be checked if necessary.
>> 288
>> 289 Clearing Requests
>> 290 -----------------
>> 291
>> 292 Generally it only makes sense for the receiving VCPU thread to clear a
>> 293 request. However, in some circumstances, such as when the requesting
>> 294 thread and the receiving VCPU thread are executed serially, such as when
>> 295 they are the same thread, or when they are using some form of concurrency
>> 296 control to temporarily execute synchronously, then it's possible to know
>> 297 that the request may be cleared immediately, rather than waiting for the
>> 298 receiving VCPU thread to handle the request in VCPU RUN. The only current
>> 299 examples of this are kvm_vcpu_block() calls made by VCPUs to block
>> 300 themselves. A possible side-effect of that call is to make the
>> 301 KVM_REQ_UNHALT request, which may then be cleared immediately when the
>> 302 VCPU returns from the call.
288 303
289 References 304 References
290 ========== 305 ==========
291 306
292 .. [atomic-ops] Documentation/atomic_bitops.tx 307 .. [atomic-ops] Documentation/atomic_bitops.txt and Documentation/atomic_t.txt
293 .. [memory-barriers] Documentation/memory-barr 308 .. [memory-barriers] Documentation/memory-barriers.txt
294 .. [lwn-mb] https://lwn.net/Articles/573436/ 309 .. [lwn-mb] https://lwn.net/Articles/573436/
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