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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Wanpeng Li <kernellwp@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH v2 1/2] KVM: VMX: FIXED+PHYSICAL mode single target IPI fastpath
In-Reply-To: <CANRm+CzcWDvRA0+iaQZ6hd2HGRKyZpRnurghQXdagDCffKaSPg@mail.gmail.com>
References: <1574145389-12149-1-git-send-email-wanpengli@tencent.com> <87r224gjyt.fsf@vitty.brq.redhat.com> <CANRm+CzcWDvRA0+iaQZ6hd2HGRKyZpRnurghQXdagDCffKaSPg@mail.gmail.com>
Date:   Tue, 19 Nov 2019 13:26:51 +0100
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Wanpeng Li <kernellwp@gmail.com> writes:

> On Tue, 19 Nov 2019 at 19:54, Vitaly Kuznetsov <vkuznets@redhat.com> wrot=
e:
>>
>> Wanpeng Li <kernellwp@gmail.com> writes:
>>
>> > From: Wanpeng Li <wanpengli@tencent.com>
>> >
>> > +     if (lapic_in_kernel(vcpu) && apic_x2apic_mode(vcpu->arch.apic)) =
{
>> > +             /*
>> > +              * fastpath to IPI target, FIXED+PHYSICAL which is popul=
ar
>> > +              */
>> > +             index =3D kvm_rcx_read(vcpu);
>> > +             data =3D kvm_read_edx_eax(vcpu);
>> > +
>> > +             if (((index - APIC_BASE_MSR) << 4 =3D=3D APIC_ICR) &&
>>
>> What if index (RCX) is < APIC_BASE_MSR?
>
> How about if (index =3D=3D (APIC_BASE_MSR + 0x300) &&
>

What about ' << 4', don't we still need it? :-) And better APIC_ICR
instead of 0x300...

Personally, I'd write something like

if (index > APIC_BASE_MSR && (index - APIC_BASE_MSR) =3D=3D APIC_ICR >> 4)

and let compiler optimize this, I bet it's going to be equally good.

--=20
Vitaly