linux-kernel - Re: [PATCH 2/2] KVM: LAPIC: reset TMCCT during vCPU reset

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Date:   Thu, 3 Jun 2021 15:34:36 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH 2/2] KVM: LAPIC: reset TMCCT during vCPU reset

On Thu, Jun 03, 2021, Jim Mattson wrote:
> On Thu, Jun 3, 2021 at 2:01 AM Wanpeng Li <kernellwp@...il.com> wrote:
> >
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > The value of current counter register after reset is 0 for both Intel
> > and AMD, let's do it in kvm.
> >
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> 
> How did we miss that?

I suspect it's not actually a functional issue, and that writing '0' at reset is
a glorified nop.  The TMCCT is always computed on-demand and never directly
readable.

Is there an observable bug being fixed?  If not, the changelog should state that
this is a cosmetic change of sorts.

static u32 __apic_read(struct kvm_lapic *apic, unsigned int offset)
{
	u32 val = 0;

	if (offset >= LAPIC_MMIO_LENGTH)
		return 0;

	switch (offset) {
	case APIC_ARBPRI:
		break;

	case APIC_TMCCT:	/* Timer CCR */
		if (apic_lvtt_tscdeadline(apic))
			return 0;

		val = apic_get_tmcct(apic);
		break;
	...
}


static u32 apic_get_tmcct(struct kvm_lapic *apic)
{
	ktime_t remaining, now;
	s64 ns;
	u32 tmcct;

	ASSERT(apic != NULL);

	/* if initial count is 0, current count should also be 0 */
	if (kvm_lapic_get_reg(apic, APIC_TMICT) == 0 ||  <------------
		apic->lapic_timer.period == 0)
		return 0;

	now = ktime_get();
	remaining = ktime_sub(apic->lapic_timer.target_expiration, now);
	if (ktime_to_ns(remaining) < 0)
		remaining = 0;

	ns = mod_64(ktime_to_ns(remaining), apic->lapic_timer.period);
	tmcct = div64_u64(ns,
			 (APIC_BUS_CYCLE_NS * apic->divide_count));

	return tmcct;
}

int kvm_apic_get_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
{
	memcpy(s->regs, vcpu->arch.apic->regs, sizeof(*s));

	/*
	 * Get calculated timer current count for remaining timer period (if
	 * any) and store it in the returned register set.
	 */
	__kvm_lapic_set_reg(s->regs, APIC_TMCCT,
			    __apic_read(vcpu->arch.apic, APIC_TMCCT));  <----

	return kvm_apic_state_fixup(vcpu, s, false);
}