linux-kernel - Re: [PATCH v2 13/23] KVM: x86: Disable APIC logical map if vCPUs are aliased in logical mode

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Message-ID: <YyGF6T/N1l3i7Ded@google.com>
Date:   Wed, 14 Sep 2022 07:42:33 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Suthikulpanit, Suravee" <suravee.suthikulpanit@....com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>,
        Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH v2 13/23] KVM: x86: Disable APIC logical map if vCPUs are
 aliased in logical mode

On Tue, Sep 13, 2022, Suthikulpanit, Suravee wrote:
> Hi Sean
> 
> On 9/2/2022 7:22 PM, Sean Christopherson wrote:
> > Disable the optimized APIC logical map if multiple vCPUs are aliased to
> > the same logical ID.  Architecturally, all CPUs whose logical ID matches
> > the MDA are supposed to receive the interrupt; overwriting existing map
> > entries can result in missed IPIs.
> > 
> > Fixes: 1e08ec4a130e ("KVM: optimize apic interrupt delivery")
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> > ---
> >   arch/x86/kvm/lapic.c | 5 +++--
> >   1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 6b2f538b8fd0..75748c380ceb 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -303,12 +303,13 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
> >   		if (!mask)
> >   			continue;
> > -		if (!is_power_of_2(mask)) {
> > +		ldr = ffs(mask) - 1;
> > +		if (!is_power_of_2(mask) || cluster[ldr]) {
> 
> Should this be checking if the cluster[ldr] is pointing to the same struct
> apic instead? For example:
> 
> 		if (!is_power_of_2(mask) || cluster[ldr] != apic)
> 
> From my observation, the kvm_recalculate_apic_map() can be called many
> times, and the cluster[ldr] could have already been assigned from the
> previous invocation. So, as long as it is the same, it should be okay.

No, because cluster[ldr] can never match "apic".  kvm_recalculate_apic_map()
creates and populates a _new_ kvm_apic_map every time, it doesn't do an in-place
update of the current map.

The loop containing this code is:

	kvm_for_each_vcpu(i, vcpu, kvm) {
		struct kvm_lapic *apic = vcpu->arch.apic;

		...
	}

so it's impossible for cluster[ldr] to hold the current "apic", because this is
the first and only iteration that processes the current "apic".