Re: [patch 4/8] KVM: x86: replace hrtimer based timer emulation

From: Marcelo Tosatti <mtosatti@redhat.com>
To: Gleb Natapov <gleb@redhat.com>
Cc: kvm@vger.kernel.org
Subject: Re: [patch 4/8] KVM: x86: replace hrtimer based timer emulation
Date: Wed, 8 Jul 2009 12:42:52 -0300	[thread overview]
Message-ID: <20090708154252.GA6364@amt.cnet> (raw)
In-Reply-To: <20090708133956.GP28046@redhat.com>

On Wed, Jul 08, 2009 at 04:39:56PM +0300, Gleb Natapov wrote:
> On Wed, Jul 08, 2009 at 10:17:21AM -0300, Marcelo Tosatti wrote:
> > On Wed, Jul 08, 2009 at 03:58:19PM +0300, Gleb Natapov wrote:
> > > Excellent patch series.
> > > 
> > > On Sun, Jul 05, 2009 at 10:55:15PM -0300, Marcelo Tosatti wrote:
> > > >  int kvm_cpu_has_pending_timer(struct kvm_vcpu *vcpu)
> > > >  {
> > > > -	int ret;
> > > > +	ktime_t now, expires;
> > > >  
> > > > -	ret = pit_has_pending_timer(vcpu);
> > > > -	ret |= apic_has_pending_timer(vcpu);
> > > > +	expires = kvm_vcpu_next_timer_event(vcpu);
> > > > +	now = ktime_get();
> > > > +	if (expires.tv64 <= now.tv64) {
> > > > +		if (kvm_arch_interrupt_allowed(vcpu))
> > > > +			set_bit(KVM_REQ_UNHALT, &vcpu->requests);
> > > You shouldn't unhalt vcpu here. Not every timer event will generate
> > > interrupt (vector can be masked in pic/ioapic)
> > 
> > Yeah. Note however that kvm_vcpu_next_timer_event only returns the
> > expiration time for events that have been acked (for timers that have
> > had events injected, but not acked it returns KTIME_MAX).
> > 
> > So, the code above will set one spurious unhalt if:
> > 
> > - inject timer irq
> > - guest acks irq
> > - guest mask irq
> > - unhalt (once)
> > 
> > I had a "kvm_timer_mask" callback before (along with the attached
> > patch), but decided to keep it simpler using the ack trick above.
> > 
> > I suppose one spurious unhalt is harmless, or is it a correctness issue?
> > 
> This is correctness issue. We should be as close to real CPU as
> possible. This will save us may hours of debugging later :)

Hum, fine. Will update the kvm_timer_mask patch below and let you know.

> > > or timer event can generate NMI instead of interrupt.
> > 
> > In the NMI case it should not unhalt the processor?
> Why? It should. It should jump to NMI handler.

I meant unhalt as in KVM_REQ_UNHALT so vcpu_enter_guest runs.

What did you mention about ISR/IRR again?