linux-kernel - Re: [PATCH v2 2/2] KVM: LAPIC: Don't need to clear IPI delivery status in x2apic mode

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Date:   Thu, 2 Apr 2020 08:14:34 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Nadav Amit <namit@...are.com>, Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v2 2/2] KVM: LAPIC: Don't need to clear IPI delivery
 status in x2apic mode

On Thu, 2 Apr 2020 at 07:01, Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Wed, Apr 01, 2020 at 05:40:03PM +0000, Nadav Amit wrote:
> > > On Mar 31, 2020, at 11:46 PM, Wanpeng Li <kernellwp@...il.com> wrote:
> > >
> > > Cc more people,
> > > On Wed, 1 Apr 2020 at 08:35, Paolo Bonzini <pbonzini@...hat.com> wrote:
> > >> On 01/04/20 02:19, Wanpeng Li wrote:
> > >>> -             /* No delay here, so we always clear the pending bit */
> > >>> -             val &= ~(1 << 12);
> > >>> +             /* Immediately clear Delivery Status in xAPIC mode */
> > >>> +             if (!apic_x2apic_mode(apic))
> > >>> +                     val &= ~(1 << 12);
> > >>
> > >> This adds a conditional, and the old behavior was valid according to the
> > >> SDM: "software should not assume the value returned by reading the ICR
> > >> is the last written value".
> > >
> > > Nadav, Sean, what do you think?
> >
> > I do not know. But if you write a KVM unit-test, I can run it on bare-metal
> > and give you feedback about how it behaves.
>
> I agree with Paolo, clearing the bit doesn't violate the SDM.  The
> conditional is just as costly as the AND, if not more so, even for x2APIC.
>
> I would play it safe and clear the bit even in the x2APIC only path to
> avoid tripping up guest kernels that loop on the delivery status even when
> using x2APIC.

Fair enough.

    Wanpeng