linux-kernel - Re: [PATCH v5 3/3] KVM: LAPIC: Apply change to TDCR right away to the timer

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Date:   Fri, 6 Oct 2017 16:20:03 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v5 3/3] KVM: LAPIC: Apply change to TDCR right away to
 the timer

2017-10-06 22:03+0800, Wanpeng Li:
> 2017-10-06 21:03 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> > 2017-10-06 07:14+0800, Wanpeng Li:
> >> 2017-10-06 2:14 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> >> > 2017-10-05 07:35-0700, Wanpeng Li:
> >> >> From: Wanpeng Li <wanpeng.li@...mail.com>
> >> >> +             remaining = ktime_sub(apic->lapic_timer.target_expiration, now);
> >> >> +             if (ktime_to_ns(remaining) < 0)
> >> >> +                     remaining = 0;
> >> >> +             delta = mod_64(ktime_to_ns(remaining), apic->lapic_timer.period);
> >> >> +
> >> >> +             if (!delta)
> >> >> +                     return false;
> >> >> +
> >> >> +             apic->lapic_timer.period = (u64)kvm_lapic_get_reg(apic, APIC_TMICT)
> >> >> +                     * APIC_BUS_CYCLE_NS * apic->divide_count;
> >> >
> >> > I'd prefer to apply the rate limiting (done earlier in this function) to
> >> > the period.  This version allows the guest to configure 128 times more
> >> > frequent interrupts in the host.
> >> > (And thinking about it, the version of [2/3] I proposed has similar
> >> >  problem when switching from one-shot to periodic, only there it is
> >> >  unpredictably limited by the speed of KVM.)
> >>
> >> We didn't stop and restart the timer, why the rate will influence us for [2/3]?
> >
> > It is because of the rate limiting -- the guest could setup a one-shot
> > timer with a short expiration and switch to periodic
> 
> Yeah, in addition, I think configure 128 means more slower interrupts
> instead of faster.

Yes, it says how many cycles it takes to decrement APIC_TMCCT.

(I only concerned about the case where rate limit was configured with
 divide_count=128 and then switched to 1.)