Message-ID: <20100825220134.GA3322@amt.cnet>
Date:	Wed, 25 Aug 2010 19:01:34 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Zachary Amsden <zamsden@...hat.com>
Cc:	kvm@...r.kernel.org, Avi Kivity <avi@...hat.com>,
	Glauber Costa <glommer@...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 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."

> Partly, the lack of implementation is by deliberate choice; the
> logic involved with setting such bounds and wisdom of doing so is a
> choice most likely to be done by a policy agent in userspace, in our
> case, qemu.  In the end, that is what has full control over the
> setting or not of guest TSC rate and choice of TSC mode.
> 
> What's lacking is the ability to force the use of a certain mode.  I
> think it's clear now, that needs to be a per-VM choice, not a global
> one.
> 
> Zach
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