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 .. SPDX-License-Identifier: GPL-2.0
 
 =================
 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_VM_DEAD
 
   This request informs all VCPUs that the VM is dead and unusable, e.g. due to
   fatal error or because the VM's state has been intentionally destroyed.
 
 KVM_REQ_UNBLOCK
 
   This request informs the vCPU to exit kvm_vcpu_block.  It is used for
   example from timer handlers that run on the host on behalf of a vCPU,
   or in order to update the interrupt routing and ensure that assigned
   devices will wake up the vCPU.
 
 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_REQ_OUTSIDE_GUEST_MODE
 
   This "request" ensures the target vCPU has exited guest mode prior to the
   sender of the request continuing on.  No action needs be taken by the target,
   and so no request is actually logged for the target.  This request is similar
   to a "kick", but unlike a kick it guarantees the vCPU has actually exited
   guest mode.  A kick only guarantees the vCPU will exit at some point in the
   future, e.g. a previous kick may have started the process, but there's no
   guarantee the to-be-kicked vCPU has fully exited guest mode.
 
 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/atomic_bitops.txt and Documentation/atomic_t.txt
 .. [memory-barriers] Documentation/memory-barriers.txt
 .. [lwn-mb] https://lwn.net/Articles/573436/

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