linux-kernel - Re: [PATCH 10/19] KVM: SVM: Document that vCPU ID == APIC ID in AVIC kick fastpatch

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Date:   Wed, 31 Aug 2022 20:49:03 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH 10/19] KVM: SVM: Document that vCPU ID == APIC ID in
 AVIC kick fastpatch

On Wed, 2022-08-31 at 16:16 +0000, Sean Christopherson wrote:
> On Wed, Aug 31, 2022, Maxim Levitsky wrote:
> > On Wed, 2022-08-31 at 00:34 +0000, Sean Christopherson wrote:
> > > Document that AVIC is inhibited if any vCPU's APIC ID diverges from its
> > > vCPU ID, i.e. that there's no need to check for a destination match in
> > > the AVIC kick fast path.
> > > 
> > > Opportunistically tweak comments to remove "guest bug", as that suggests
> > > KVM is punting on error handling, which is not the case.  Targeting a
> > > non-existent vCPU or no vCPUs _may_ be a guest software bug, but whether
> > > or not it's a guest bug is irrelevant.  Such behavior is architecturally
> > > legal and thus needs to faithfully emulated by KVM (and it is).
> > 
> > I don't want to pick a fight,
> 
> Please don't hesitate to push back, I would much rather discuss points of contention
> than have an ongoing, silent battle through patches.
> 
> > but personally these things *are* guest bugs / improper usage of APIC, and I
> > don't think it is wrong to document them as such.
> 
> If the guest is intentionally exercising an edge case, e.g. KUT or selftests, then
> from the guest's perspective its code/behavior isn't buggy.
> 
> I completely agree that abusing/aliasing logical IDs is improper usage and arguably
> out of spec, but the scenarios here are very much in spec, e.g. a bitmap of '0'
> isn't expressly forbidden and both Intel and AMD specs very clearly state that
> APICs discard interrupt messages if they are not the destination.
> 
> But that's somewhat beside the point, as I have no objection to documenting scenarios
> that are out-of-spec or undefined.  What I object to is documenting such scenarios as
> "guest bugs".  If the CPU/APIC/whatever behavior is undefined, then document it
> as undefined.  Saying "guest bug" doesn't help future readers understand what is
> architecturally supposed to happen, whereas a comment like
> 
> 	/*
> 	 * Logical IDs are architecturally "required" to be unique, i.e. this is
> 	 * technically undefined behavior.  Disable the optimized logical map so
> 	 * that KVM is consistent with itself, as the non-optimized matching
> 	 * logic with accept interrupts on all CPUs with the logical ID.
> 	 */
> 
> anchors KVM's behavior to the specs and explains why KVM does XYZ in response to
> undefined behavior.
> 
> I feel very strongly about "guest bug" because KVM has a history of disregarding
> architectural correctness and using a "good enough" approach.  Simply stating
> "guest bug" makes it all the more difficult to differentiate between KVM handling
> architecturally undefined behavior, versus KVM deviating from the spec because
> someone decided that KVM's partial emulation/virtualziation was "good enough".
> 

All right, I agree with you.

Best regards,
	Maxim Levitsky