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    SubjectRe: [PATCH RFC V6 1/5] kvm hypervisor : Add a hypercall to KVM hypervisor to support pv-ticketlocks
    On Mon, Apr 30, 2012 at 11:22:34AM +0300, Avi Kivity wrote:
    > On 04/29/2012 04:52 PM, Gleb Natapov wrote:
    > > On Sun, Apr 29, 2012 at 04:26:21PM +0300, Avi Kivity wrote:
    > > > On 04/29/2012 04:20 PM, Gleb Natapov wrote:
    > > > > > > This is too similar to kvm_irq_delivery_to_apic(). Why not reuse it. We
    > > > > > > can use one of reserved delivery modes as PV delivery mode. We will
    > > > > > > disallow guest to trigger it through apic interface, so this will not be
    > > > > > > part of ABI and can be changed at will.
    > > > > > >
    > > > > >
    > > > > > I'm not thrilled about this. Those delivery modes will eventually
    > > > > > become unreserved. We can have a kvm_lookup_apic_id() that is shared
    > > > > > among implementations.
    > > > > >
    > > > > This is only internal implementation. If they become unreserved we will
    > > > > use something else.
    > > > >
    > > >
    > > > Yeah, I'm thinking of that time. Why do something temporary and fragile?
    > > >
    > > Why is it fragile? Just by unreserving the value Intel will not break
    > > KVM. Only when KVM will implement apic feature that unreserves the value
    > > we will have to change internal implementation and use another value,
    > > but this will be done by the same patch that does unreserving. The
    > > unreserving may even never happen.
    >
    > Some remains of that may leak somewhere.
    I do not see where. APIC code should #GP if a guest attempts to set
    reserved values through APIC interface, or at least ignore them.

    > Why not add an extra
    > parameter?
    Yes, we can add extra parameter to "struct kvm_lapic_irq" and propagate it to
    __apic_accept_irq(). We can do that now, or when Intel unreserve all
    reserved values. I prefer the later since I do not believe it will
    happen in observable feature.

    > Or do something like
    >
    > kvm_for_each_apic_dest(vcpu, apic_destination) {
    > ...
    > }
    >
    > That can be reused in both the apic code and pv kick.
    >
    That's exactly what kvm_irq_delivery_to_apic() is.

    > > Meanwhile kvm_irq_delivery_to_apic()
    > > will likely be optimized to use hash for unicast delivery and unhalt
    > > hypercall will benefit from it immediately.
    >
    > Overloading delivery mode is not the only way to achieve sharing.
    >
    It is simplest and most straightforward with no demonstratable drawbacks :)
    Adding parameter to "struct kvm_lapic_irq" is next best thing.

    --
    Gleb.


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