KVM: Add documentation for VCPU requests

Signed-off-by: NAndrew Jones <drjones@redhat.com>
Acked-by: NChristoffer Dall <cdall@linaro.org>
Signed-off-by: NChristoffer Dall <cdall@linaro.org>

Documentation/virtual/kvm/vcpu-requests.rst

+=================
+KVM VCPU Requests
+=================
+
+Overview
+========
+
+KVM supports an internal API enabling threads to request a VCPU thread to
+perform some activity.  For example, a thread may request a VCPU to flush
+its TLB with a VCPU request.  The API consists of the following functions::
+
+  /* Check if any requests are pending for VCPU @vcpu. */
+  bool kvm_request_pending(struct kvm_vcpu *vcpu);
+
+  /* Check if VCPU @vcpu has request @req pending. */
+  bool kvm_test_request(int req, struct kvm_vcpu *vcpu);
+
+  /* Clear request @req for VCPU @vcpu. */
+  void kvm_clear_request(int req, struct kvm_vcpu *vcpu);
+
+  /*
+   * Check if VCPU @vcpu has request @req pending. When the request is
+   * pending it will be cleared and a memory barrier, which pairs with
+   * another in kvm_make_request(), will be issued.
+   */
+  bool kvm_check_request(int req, struct kvm_vcpu *vcpu);
+
+  /*
+   * Make request @req of VCPU @vcpu. Issues a memory barrier, which pairs
+   * with another in kvm_check_request(), prior to setting the request.
+   */
+  void kvm_make_request(int req, struct kvm_vcpu *vcpu);
+
+  /* Make request @req of all VCPUs of the VM with struct kvm @kvm. */
+  bool kvm_make_all_cpus_request(struct kvm *kvm, unsigned int req);
+
+Typically a requester wants the VCPU to perform the activity as soon
+as possible after making the request.  This means most requests
+(kvm_make_request() calls) are followed by a call to kvm_vcpu_kick(),
+and kvm_make_all_cpus_request() has the kicking of all VCPUs built
+into it.
+
+VCPU Kicks
+----------
+
+The goal of a VCPU kick is to bring a VCPU thread out of guest mode in
+order to perform some KVM maintenance.  To do so, an IPI is sent, forcing
+a guest mode exit.  However, a VCPU thread may not be in guest mode at the
+time of the kick.  Therefore, depending on the mode and state of the VCPU
+thread, there are two other actions a kick may take.  All three actions
+are listed below:
+
+1) Send an IPI.  This forces a guest mode exit.
+2) Waking a sleeping VCPU.  Sleeping VCPUs are VCPU threads outside guest
+   mode that wait on waitqueues.  Waking them removes the threads from
+   the waitqueues, allowing the threads to run again.  This behavior
+   may be suppressed, see KVM_REQUEST_NO_WAKEUP below.
+3) Nothing.  When the VCPU is not in guest mode and the VCPU thread is not
+   sleeping, then there is nothing to do.
+
+VCPU Mode
+---------
+
+VCPUs have a mode state, ``vcpu->mode``, that is used to track whether the
+guest is running in guest mode or not, as well as some specific
+outside guest mode states.  The architecture may use ``vcpu->mode`` to
+ensure VCPU requests are seen by VCPUs (see "Ensuring Requests Are Seen"),
+as well as to avoid sending unnecessary IPIs (see "IPI Reduction"), and
+even to ensure IPI acknowledgements are waited upon (see "Waiting for
+Acknowledgements").  The following modes are defined:
+
+OUTSIDE_GUEST_MODE
+
+  The VCPU thread is outside guest mode.
+
+IN_GUEST_MODE
+
+  The VCPU thread is in guest mode.
+
+EXITING_GUEST_MODE
+
+  The VCPU thread is transitioning from IN_GUEST_MODE to
+  OUTSIDE_GUEST_MODE.
+
+READING_SHADOW_PAGE_TABLES
+
+  The VCPU thread is outside guest mode, but it wants the sender of
+  certain VCPU requests, namely KVM_REQ_TLB_FLUSH, to wait until the VCPU
+  thread is done reading the page tables.
+
+VCPU Request Internals
+======================
+
+VCPU requests are simply bit indices of the ``vcpu->requests`` bitmap.
+This means general bitops, like those documented in [atomic-ops]_ could
+also be used, e.g. ::
+
+  clear_bit(KVM_REQ_UNHALT & KVM_REQUEST_MASK, &vcpu->requests);
+
+However, VCPU request users should refrain from doing so, as it would
+break the abstraction.  The first 8 bits are reserved for architecture
+independent requests, all additional bits are available for architecture
+dependent requests.
+
+Architecture Independent Requests
+---------------------------------
+
+KVM_REQ_TLB_FLUSH
+
+  KVM's common MMU notifier may need to flush all of a guest's TLB
+  entries, calling kvm_flush_remote_tlbs() to do so.  Architectures that
+  choose to use the common kvm_flush_remote_tlbs() implementation will
+  need to handle this VCPU request.
+
+KVM_REQ_MMU_RELOAD
+
+  When shadow page tables are used and memory slots are removed it's
+  necessary to inform each VCPU to completely refresh the tables.  This
+  request is used for that.
+
+KVM_REQ_PENDING_TIMER
+
+  This request may be made from a timer handler run on the host on behalf
+  of a VCPU.  It informs the VCPU thread to inject a timer interrupt.
+
+KVM_REQ_UNHALT
+
+  This request may be made from the KVM common function kvm_vcpu_block(),
+  which is used to emulate an instruction that causes a CPU to halt until
+  one of an architectural specific set of events and/or interrupts is
+  received (determined by checking kvm_arch_vcpu_runnable()).  When that
+  event or interrupt arrives kvm_vcpu_block() makes the request.  This is
+  in contrast to when kvm_vcpu_block() returns due to any other reason,
+  such as a pending signal, which does not indicate the VCPU's halt
+  emulation should stop, and therefore does not make the request.
+
+KVM_REQUEST_MASK
+----------------
+
+VCPU requests should be masked by KVM_REQUEST_MASK before using them with
+bitops.  This is because only the lower 8 bits are used to represent the
+request's number.  The upper bits are used as flags.  Currently only two
+flags are defined.
+
+VCPU Request Flags
+------------------
+
+KVM_REQUEST_NO_WAKEUP
+
+  This flag is applied to requests that only need immediate attention
+  from VCPUs running in guest mode.  That is, sleeping VCPUs do not need
+  to be awaken for these requests.  Sleeping VCPUs will handle the
+  requests when they are awaken later for some other reason.
+
+KVM_REQUEST_WAIT
+
+  When requests with this flag are made with kvm_make_all_cpus_request(),
+  then the caller will wait for each VCPU to acknowledge its IPI before
+  proceeding.  This flag only applies to VCPUs that would receive IPIs.
+  If, for example, the VCPU is sleeping, so no IPI is necessary, then
+  the requesting thread does not wait.  This means that this flag may be
+  safely combined with KVM_REQUEST_NO_WAKEUP.  See "Waiting for
+  Acknowledgements" for more information about requests with
+  KVM_REQUEST_WAIT.
+
+VCPU Requests with Associated State
+===================================
+
+Requesters that want the receiving VCPU to handle new state need to ensure
+the newly written state is observable to the receiving VCPU thread's CPU
+by the time it observes the request.  This means a write memory barrier
+must be inserted after writing the new state and before setting the VCPU
+request bit.  Additionally, on the receiving VCPU thread's side, a
+corresponding read barrier must be inserted after reading the request bit
+and before proceeding to read the new state associated with it.  See
+scenario 3, Message and Flag, of [lwn-mb]_ and the kernel documentation
+[memory-barriers]_.
+
+The pair of functions, kvm_check_request() and kvm_make_request(), provide
+the memory barriers, allowing this requirement to be handled internally by
+the API.
+
+Ensuring Requests Are Seen
+==========================
+
+When making requests to VCPUs, we want to avoid the receiving VCPU
+executing in guest mode for an arbitrary long time without handling the
+request.  We can be sure this won't happen as long as we ensure the VCPU
+thread checks kvm_request_pending() before entering guest mode and that a
+kick will send an IPI to force an exit from guest mode when necessary.
+Extra care must be taken to cover the period after the VCPU thread's last
+kvm_request_pending() check and before it has entered guest mode, as kick
+IPIs will only trigger guest mode exits for VCPU threads that are in guest
+mode or at least have already disabled interrupts in order to prepare to
+enter guest mode.  This means that an optimized implementation (see "IPI
+Reduction") must be certain when it's safe to not send the IPI.  One
+solution, which all architectures except s390 apply, is to:
+
+- set ``vcpu->mode`` to IN_GUEST_MODE between disabling the interrupts and
+  the last kvm_request_pending() check;
+- enable interrupts atomically when entering the guest.
+
+This solution also requires memory barriers to be placed carefully in both
+the requesting thread and the receiving VCPU.  With the memory barriers we
+can exclude the possibility of a VCPU thread observing
+!kvm_request_pending() on its last check and then not receiving an IPI for
+the next request made of it, even if the request is made immediately after
+the check.  This is done by way of the Dekker memory barrier pattern
+(scenario 10 of [lwn-mb]_).  As the Dekker pattern requires two variables,
+this solution pairs ``vcpu->mode`` with ``vcpu->requests``.  Substituting
+them into the pattern gives::
+
+  CPU1                                    CPU2
+  =================                       =================
+  local_irq_disable();
+  WRITE_ONCE(vcpu->mode, IN_GUEST_MODE);  kvm_make_request(REQ, vcpu);
+  smp_mb();                               smp_mb();
+  if (kvm_request_pending(vcpu)) {        if (READ_ONCE(vcpu->mode) ==
+                                              IN_GUEST_MODE) {
+      ...abort guest entry...                 ...send IPI...
+  }                                       }
+
+As stated above, the IPI is only useful for VCPU threads in guest mode or
+that have already disabled interrupts.  This is why this specific case of
+the Dekker pattern has been extended to disable interrupts before setting
+``vcpu->mode`` to IN_GUEST_MODE.  WRITE_ONCE() and READ_ONCE() are used to
+pedantically implement the memory barrier pattern, guaranteeing the
+compiler doesn't interfere with ``vcpu->mode``'s carefully planned
+accesses.
+
+IPI Reduction
+-------------
+
+As only one IPI is needed to get a VCPU to check for any/all requests,
+then they may be coalesced.  This is easily done by having the first IPI
+sending kick also change the VCPU mode to something !IN_GUEST_MODE.  The
+transitional state, EXITING_GUEST_MODE, is used for this purpose.
+
+Waiting for Acknowledgements
+----------------------------
+
+Some requests, those with the KVM_REQUEST_WAIT flag set, require IPIs to
+be sent, and the acknowledgements to be waited upon, even when the target
+VCPU threads are in modes other than IN_GUEST_MODE.  For example, one case
+is when a target VCPU thread is in READING_SHADOW_PAGE_TABLES mode, which
+is set after disabling interrupts.  To support these cases, the
+KVM_REQUEST_WAIT flag changes the condition for sending an IPI from
+checking that the VCPU is IN_GUEST_MODE to checking that it is not
+OUTSIDE_GUEST_MODE.
+
+Request-less VCPU Kicks
+-----------------------
+
+As the determination of whether or not to send an IPI depends on the
+two-variable Dekker memory barrier pattern, then it's clear that
+request-less VCPU kicks are almost never correct.  Without the assurance
+that a non-IPI generating kick will still result in an action by the
+receiving VCPU, as the final kvm_request_pending() check does for
+request-accompanying kicks, then the kick may not do anything useful at
+all.  If, for instance, a request-less kick was made to a VCPU that was
+just about to set its mode to IN_GUEST_MODE, meaning no IPI is sent, then
+the VCPU thread may continue its entry without actually having done
+whatever it was the kick was meant to initiate.
+
+One exception is x86's posted interrupt mechanism.  In this case, however,
+even the request-less VCPU kick is coupled with the same
+local_irq_disable() + smp_mb() pattern described above; the ON bit
+(Outstanding Notification) in the posted interrupt descriptor takes the
+role of ``vcpu->requests``.  When sending a posted interrupt, PIR.ON is
+set before reading ``vcpu->mode``; dually, in the VCPU thread,
+vmx_sync_pir_to_irr() reads PIR after setting ``vcpu->mode`` to
+IN_GUEST_MODE.
+
+Additional Considerations
+=========================
+
+Sleeping VCPUs
+--------------
+
+VCPU threads may need to consider requests before and/or after calling
+functions that may put them to sleep, e.g. kvm_vcpu_block().  Whether they
+do or not, and, if they do, which requests need consideration, is
+architecture dependent.  kvm_vcpu_block() calls kvm_arch_vcpu_runnable()
+to check if it should awaken.  One reason to do so is to provide
+architectures a function where requests may be checked if necessary.
+
+Clearing Requests
+-----------------
+
+Generally it only makes sense for the receiving VCPU thread to clear a
+request.  However, in some circumstances, such as when the requesting
+thread and the receiving VCPU thread are executed serially, such as when
+they are the same thread, or when they are using some form of concurrency
+control to temporarily execute synchronously, then it's possible to know
+that the request may be cleared immediately, rather than waiting for the
+receiving VCPU thread to handle the request in VCPU RUN.  The only current
+examples of this are kvm_vcpu_block() calls made by VCPUs to block
+themselves.  A possible side-effect of that call is to make the
+KVM_REQ_UNHALT request, which may then be cleared immediately when the
+VCPU returns from the call.
+
+References
+==========
+
+.. [atomic-ops] Documentation/core-api/atomic_ops.rst
+.. [memory-barriers] Documentation/memory-barriers.txt
+.. [lwn-mb] https://lwn.net/Articles/573436/