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

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Message-ID: <6de1a454-60fc-2bda-841d-f9ceb606d4c6@redhat.com>
Date:   Wed, 1 Apr 2020 02:35:33 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        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 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".

Paolo