On Tue, Dec 11, 2018 at 01:45:05PM +0100, Vitaly Kuznetsov wrote:
> Roman Kagan <rkagan@xxxxxxxxxxxxx> writes:
>
> > The SynIC message delivery protocol allows the message originator to
> > request, should the message slot be busy, to be notified when it's free.
> >
> > However, this is unnecessary and even undesirable for messages generated
> > by SynIC timers in periodic mode: if the period is short enough compared
> > to the time the guest spends in the timer interrupt handler, so the
> > timer ticks start piling up, the excessive interactions due to this
> > notification and retried message delivery only makes the things worse.
> >
> > [This was observed, in particular, with Windows L2 guests setting
> > (temporarily) the periodic timer to 2 kHz, and spending hundreds of
> > microseconds in the timer interrupt handler due to several L2->L1 exits;
> > under some load in L0 this could exceed 500 us so the timer ticks
> > started to pile up and the guest livelocked.]
> >
> > Relieve the situation somewhat by not retrying message delivery for
> > periodic SynIC timers. This appears to remain within the "lazy" lost
> > ticks policy for SynIC timers as implemented in KVM.
> >
> > Note that it doesn't solve the fundamental problem of livelocking the
> > guest with a periodic timer whose period is smaller than the time needed
> > to process a tick, but it makes it a bit less likely to be triggered.
> >
>
> We seem to be ignoring 'lazy' setting for stimers completely; Going
> forward, do you think it would make sense to try to implement both
> lazy and non-lazy modes for stimers as they're described in TLFS
> (basically, we need to implement 'non-lazy' mode)?
The alternative to "lazy" is "catch-up". I guess it's only relevant to
guests that do timekeeping by tick counting. Hopefully no sane guest
does it. Especially so since we're talking about paravirtualized
interfaces, and the guests using them should be aware that tick counting
can't be relied upon when running under hypervisor.
> (now I'm interested in which mode Windows and Hyper-V actually use).
>
> > Signed-off-by: Roman Kagan <rkagan@xxxxxxxxxxxxx>
> > ---
> > arch/x86/kvm/hyperv.c | 13 +++++++++++--
> > 1 file changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> > index 17d04d2c6d4f..3f2b93ad9ccf 100644
> > --- a/arch/x86/kvm/hyperv.c
> > +++ b/arch/x86/kvm/hyperv.c
> > @@ -557,7 +557,7 @@ static int stimer_get_count(struct kvm_vcpu_hv_stimer *stimer, u64 *pcount)
> > }
> >
> > static int synic_deliver_msg(struct kvm_vcpu_hv_synic *synic, u32 sint,
> > - struct hv_message *src_msg)
> > + struct hv_message *src_msg, bool no_retry)
> > {
> > struct kvm_vcpu *vcpu = synic_to_vcpu(synic);
> > int msg_off = offsetof(struct hv_message_page, sint_message[sint]);
> > @@ -584,6 +584,9 @@ static int synic_deliver_msg(struct kvm_vcpu_hv_synic *synic, u32 sint,
> > return r;
> >
> > if (hv_hdr.message_type != HVMSG_NONE) {
> > + if (no_retry)
> > + return 0;
> > +
> > hv_hdr.message_flags.msg_pending = 1;
> > r = kvm_vcpu_write_guest_page(vcpu, msg_page_gfn,
> > &hv_hdr.message_flags,
> > @@ -616,11 +619,17 @@ static int stimer_send_msg(struct kvm_vcpu_hv_stimer *stimer)
> > struct hv_message *msg = &stimer->msg;
> > struct hv_timer_message_payload *payload =
> > (struct hv_timer_message_payload *)&msg->u.payload;
> > + /*
> > + * don't retry message delivery for periodic ticks, otherwise they may
> > + * pile up
> > + */
>
> Capital 'D' and hard stop at the end would be appreciated :-) Also, it
> may make sense to mention our 'laziness' here.
/*
* To avoid piling up periodic ticks, don't retry message
* delivery for them (within "lazy" lost ticks policy).
*/
?
>
> > + bool no_retry = stimer->config & HV_STIMER_PERIODIC;
> >
> > payload->expiration_time = stimer->exp_time;
> > payload->delivery_time = get_time_ref_counter(vcpu->kvm);
> > return synic_deliver_msg(vcpu_to_synic(vcpu),
> > - HV_STIMER_SINT(stimer->config), msg);
> > + HV_STIMER_SINT(stimer->config), msg,
> > + no_retry);
> > }
> >
> > static void stimer_expiration(struct kvm_vcpu_hv_stimer *stimer)
>
> No mater how hard we try missing ticks for periodic timers is inevitable
> and TLFS states that for both lazy and non-lazy modes. As we seem to be
> implementing 'lazy' mode for now I don't envision breakages.
>
> Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Thanks,
Roman.
