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

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Date:   Tue, 8 Jun 2021 08:31:02 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Jim Mattson <jmattson@...gle.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, 3 Jun 2021 at 23:34, Sean Christopherson <seanjc@...gle.com> wrote:
>
> 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.

Update the patch description in v2, thanks.

    Wanpeng

>
> 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);
> }
>
>
>