linux-kernel - Re: [PATCH 1/2] KVM: LAPIC: Prevent setting the tscdeadline timer if the lapic is hw disabled

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Date:   Tue, 21 Jul 2020 08:25:19 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH 1/2] KVM: LAPIC: Prevent setting the tscdeadline timer if
 the lapic is hw disabled

On Tue, Jul 21, 2020 at 12:35:01PM +0200, Vitaly Kuznetsov wrote:
> Wanpeng Li <kernellwp@...il.com> writes:
> 
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > Prevent setting the tscdeadline timer if the lapic is hw disabled.
> >
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>

A Fixes and/or Cc stable is probably needed for this.

> > ---
> >  arch/x86/kvm/lapic.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 5bf72fc..4ce2ddd 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -2195,7 +2195,7 @@ void kvm_set_lapic_tscdeadline_msr(struct kvm_vcpu *vcpu, u64 data)
> >  {
> >  	struct kvm_lapic *apic = vcpu->arch.apic;
> >  
> > -	if (!lapic_in_kernel(vcpu) || apic_lvtt_oneshot(apic) ||
> > +	if (!kvm_apic_present(vcpu) || apic_lvtt_oneshot(apic) ||
> >  			apic_lvtt_period(apic))
> >  		return;
> 
> Out of pure curiosity, what is the architectural behavior if I disable
> LAPIC, write to IA32_TSC_DEADLINE and then re-enable LAPIC before the
> timer was supposed to fire?

Intel's SDM reserves the right for the CPU to do whatever it wants :-)

   When IA32_APIC_BASE[11] is set to 0, prior initialization to the APIC
   may be lost and the APIC may return to the state described in Section
   10.4.7.1, “Local APIC State After Power-Up or Reset.”

Practically speaking, resetting APIC state seems like the sane approach,
i.e. KVM should probably call kvm_lapic_reset() when the APIC transitions
from HW enabled -> disabled.  Maybe in a follow-up patch to this one?