Re: [PATCH v3 02/10] KVM: Add documentation for VCPU requests

Andrew Jones
2017-05-04T12:06:18Z

On Thu, May 04, 2017 at 01:27:35PM +0200, Paolo Bonzini wrote:
>
>
> On 03/05/2017 18:06, Andrew Jones wrote:
> > Signed-off-by: Andrew Jones <drjones@redhat.com>
> > ---
> > Documentation/virtual/kvm/vcpu-requests.rst | 269 ++++++++++++++++++++++++++++
>
> I for one welcome our new reStructuredText overlords. :)
>
> Thanks for the excellent writeup.
>
> > 1 file changed, 269 insertions(+)
> > create mode 100644 Documentation/virtual/kvm/vcpu-requests.rst
> >
> > diff --git a/Documentation/virtual/kvm/vcpu-requests.rst b/Documentation/virtual/kvm/vcpu-requests.rst
> > new file mode 100644
> > index 000000000000..d74616d7999a
> > --- /dev/null
> > +++ b/Documentation/virtual/kvm/vcpu-requests.rst
> > @@ -0,0 +1,269 @@
> > +=================
> > +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 and wants certain VCPU requests,
> > + namely KVM_REQ_TLB_FLUSH, to be delayed until it's done reading the
> > + page tables.
>
> ... 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.

fixed

>
> > +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 reserved, and may be used as flags.
>
> The upper bits are used as flags. Currently only two flags are defined.

fixed

>
> > +VCPU Request Flags
> > +------------------
> > +
> > +KVM_REQUEST_NO_WAKEUP
> > +
> > + This flag is applied to a request that does not need immediate
> > + attention. When a request does not need immediate attention, and the
> > + VCPU's thread is outside guest mode sleeping, then the thread is not
> > + awaken by a kick.
> > +
> > +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 the IPI before
> > + proceeding.
> > +
> > +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 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 VCPU run
> > +loops 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 prior to the last
> > +kvm_request_pending() check and to rely on memory barrier guarantees.
>
> 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.
>
> Then at the beginning of the next paragraph: "This solution also
> requires memory barriers to be placed carefully in both the sender of
> the IPI and the VCPU thread."

fixed

>
> Should vcpu->mode and IN_GUEST_MODE use monospaced font? Likewise
> elsewhere in the document.

I thought about that, but all the `` that would entail might make the
text viewing a bit too ugly. I like rst because, when its features are
minimally used, it looks nice even as raw text, especially with vim's
syntax highlighting. Anyway, I'll quick add the `` to see how it looks,
but if it's too much, then I guess I'll drop them again.

>
>
> > +With 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. For these cases, the "should send an
> > +IPI" condition becomes READ_ONCE(vcpu->mode) != 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.

I'll add this paragraph you wrote.

>
> > +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.
>
> What did you have in mind here?

I was trying to point out vcpu request concerns with respect to sleeping
vcpus, but to stay as general as possible. I can't really think of
anything else to say here, other than to give some hypothetical example.
For a while I was thinking I might check requests (kvm_request_pending())
from the kvm_arch_vcpu_runnable() call for ARM, but then changed my mind
on that - leaving it only checking the pause and power_off booleans.
Anyway, I don't think the above paragraph is "wrong", but if it's
confusing then I can change / remove it as people like. Just let me know
how you'd like it changed :-)

Thanks,
drew

>
> Paolo
>
> > +References
> > +==========
> > +
> > +.. [atomic-ops] Documentation/core-api/atomic_ops.rst
> > +.. [memory-barriers] Documentation/memory-barriers.txt
> > +.. [lwn-mb] https://lwn.net/Articles/573436/
> >