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Date: Wed, 25 Aug 2010 20:38:05 -0300
From: Glauber Costa <glommer@...hat.com>
To: Marcelo Tosatti <mtosatti@...hat.com>
Cc: Zachary Amsden <zamsden@...hat.com>, kvm@...r.kernel.org,
Avi Kivity <avi@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
John Stultz <johnstul@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [KVM timekeeping 25/35] Add clock catchup mode
On Wed, Aug 25, 2010 at 07:01:34PM -0300, Marcelo Tosatti wrote:
> On Wed, Aug 25, 2010 at 10:48:20AM -1000, Zachary Amsden wrote:
> > On 08/25/2010 07:27 AM, Marcelo Tosatti wrote:
> > >On Thu, Aug 19, 2010 at 10:07:39PM -1000, Zachary Amsden wrote:
> > >>Make the clock update handler handle generic clock synchronization,
> > >>not just KVM clock. We add a catchup mode which keeps passthrough
> > >>TSC in line with absolute guest TSC.
> > >>
> > >>Signed-off-by: Zachary Amsden<zamsden@...hat.com>
> > >>---
> > >> arch/x86/include/asm/kvm_host.h | 1 +
> > >> arch/x86/kvm/x86.c | 55 ++++++++++++++++++++++++++------------
> > >> 2 files changed, 38 insertions(+), 18 deletions(-)
> > >>
> > >> kvm_x86_ops->vcpu_load(vcpu, cpu);
> > >>- if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
> > >>+ if (unlikely(vcpu->cpu != cpu) || vcpu->arch.tsc_rebase) {
> > >> /* Make sure TSC doesn't go backwards */
> > >> s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
> > >> native_read_tsc() - vcpu->arch.last_host_tsc;
> > >> if (tsc_delta< 0)
> > >> mark_tsc_unstable("KVM discovered backwards TSC");
> > >>- if (check_tsc_unstable())
> > >>+ if (check_tsc_unstable()) {
> > >> kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta);
> > >>- kvm_migrate_timers(vcpu);
> > >>+ kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> > >>+ }
> > >>+ if (vcpu->cpu != cpu)
> > >>+ kvm_migrate_timers(vcpu);
> > >> vcpu->cpu = cpu;
> > >>+ vcpu->arch.tsc_rebase = 0;
> > >> }
> > >> }
> > >>
> > >>@@ -1947,6 +1961,12 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> > >> kvm_x86_ops->vcpu_put(vcpu);
> > >> kvm_put_guest_fpu(vcpu);
> > >> vcpu->arch.last_host_tsc = native_read_tsc();
> > >>+
> > >>+ /* For unstable TSC, force compensation and catchup on next CPU */
> > >>+ if (check_tsc_unstable()) {
> > >>+ vcpu->arch.tsc_rebase = 1;
> > >>+ kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> > >>+ }
> > >The mix between catchup,trap versus stable,unstable TSC is confusing and
> > >difficult to grasp. Can you please introduce all the infrastructure
> > >first, then control usage of them in centralized places? Examples:
> > >
> > >+static void kvm_update_tsc_trapping(struct kvm *kvm)
> > >+{
> > >+ int trap, i;
> > >+ struct kvm_vcpu *vcpu;
> > >+
> > >+ trap = check_tsc_unstable()&& atomic_read(&kvm->online_vcpus)> 1;
> > >+ kvm_for_each_vcpu(i, vcpu, kvm)
> > >+ kvm_x86_ops->set_tsc_trap(vcpu, trap&& !vcpu->arch.time_page);
> > >+}
> > >
> > >+ /* For unstable TSC, force compensation and catchup on next CPU */
> > >+ if (check_tsc_unstable()) {
> > >+ vcpu->arch.tsc_rebase = 1;
> > >+ kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> > >+ }
> > >
> > >
> > >kvm_guest_time_update is becoming very confusing too. I understand this
> > >is due to the many cases its dealing with, but please make it as simple
> > >as possible.
> >
> > I tried to comment as best as I could. I think the whole
> > "kvm_update_tsc_trapping" thing is probably a poor design choice.
> > It works, but it's thoroughly unintelligible right now without
> > spending some days figuring out why.
> >
> > I'll rework the tail series of patches to try to make them more clear.
> >
> > >+ /*
> > >+ * If we are trapping and no longer need to, use catchup to
> > >+ * ensure passthrough TSC will not be less than trapped TSC
> > >+ */
> > >+ if (vcpu->tsc_mode == TSC_MODE_PASSTHROUGH&& vcpu->tsc_trapping&&
> > >+ ((this_tsc_khz<= v->kvm->arch.virtual_tsc_khz || kvmclock))) {
> > >+ catchup = 1;
> > >
> > >What, TSC trapping with kvmclock enabled?
> >
> > Transitioning to use of kvmclock after a cold boot means we may have
> > been trapping and now we will not be.
> >
> > >For both catchup and trapping the resolution of the host clock is
> > >important, as Glauber commented for kvmclock. Can you comment on the
> > >problems that arrive from a low res clock for both modes?
> > >
> > >Similarly for catchup mode, the effect of exit frequency. No need for
> > >any guarantees?
> >
> > The scheduler will do something to get an IRQ at whatever resolution
> > it uses for it's timeslice. That guarantees an exit per timeslice,
> > so we'll never be behind by more than one slice while scheduling.
> > While not scheduling, we're dormant anyway, waiting on either an IRQ
> > or shared memory variable change. Local timers could end up behind
> > when dormant.
> >
> > We may need a hack to accelerate firing of timers in such a case, or
> > perhaps bounds on when to use catchup mode and when to not.
>
> What about emulating rdtsc with low res clock?
>
> "The RDTSC instruction reads the time-stamp counter and is guaranteed to
> return a monotonically increasing unique value whenever executed, except
> for a 64-bit counter wraparound."
>
This is bad semantics, IMHO. It is a totally different behaviour than the
one guest users would expect.
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