linux-kernel - Re: [PATCH 5/5] x86/kvm: Add KVM_FEATURE_MSI_EXT_DEST

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Date:   Tue, 13 Oct 2020 11:53:04 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Marc Zyngier <maz@...nel.org>
Cc:     kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] x86/kvm: Add KVM_FEATURE_MSI_EXT_DEST_ID

On Tue, 2020-10-13 at 12:46 +0200, Thomas Gleixner wrote:
> And after becoming more awake, that wont work anyway because there is
> more than one IR domain, so there is no way to return an error "You
> forgot to register" obviously.
> 
> But the APIC id (32768) valid check is also broken because IR can be
> enabled even without X2APIC.

Nope, it's perfectly OK to allow HPET and I/OAPIC to be parented in the
x86_vector_domain in that case, regardless of IR.

The *actual* criterion for x86_vector_select() returning zero to say 
"don't use me" is "could there be CPUs in the system which can't be
reached through x86_vector_msi_compose_msg()?". It's not really about
IR at all.

The apic_id_valid(32768) check is checking precisely the right thing.

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