linux-kernel - Re: [PATCH v2 1/2] KVM: VMX: FIXED+PHYSICAL mode single target IPI fastpath

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Message-ID: <87lfscgigk.fsf@vitty.brq.redhat.com>
Date:   Tue, 19 Nov 2019 13:26:51 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v2 1/2] KVM: VMX: FIXED+PHYSICAL mode single target IPI fastpath

Wanpeng Li <kernellwp@...il.com> writes:

> On Tue, 19 Nov 2019 at 19:54, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>>
>> Wanpeng Li <kernellwp@...il.com> writes:
>>
>> > From: Wanpeng Li <wanpengli@...cent.com>
>> >
>> > +     if (lapic_in_kernel(vcpu) && apic_x2apic_mode(vcpu->arch.apic)) {
>> > +             /*
>> > +              * fastpath to IPI target, FIXED+PHYSICAL which is popular
>> > +              */
>> > +             index = kvm_rcx_read(vcpu);
>> > +             data = kvm_read_edx_eax(vcpu);
>> > +
>> > +             if (((index - APIC_BASE_MSR) << 4 == APIC_ICR) &&
>>
>> What if index (RCX) is < APIC_BASE_MSR?
>
> How about if (index == (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) == APIC_ICR >> 4)

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

-- 
Vitaly