2015-12-16 09:37+0800, Feng Wu:
> Use vector-hashing to deliver lowest-priority interrupts, As an
> example, modern Intel CPUs in server platform use this method to
> handle lowest-priority interrupts.
>
> Signed-off-by: Feng Wu <feng...@intel.com>
> ---
> diff --git a/arch/x86/kvm/irq_comm.c b/arch/x86/kvm/irq_comm.c
> @@ -78,13 +83,25 @@ int kvm_irq_delivery_to_apic(struct kvm *kvm, struct
> kvm_lapic *src,
> r = 0;
> r += kvm_apic_set_irq(vcpu, irq, dest_map);
> } else if (kvm_lapic_enabled(vcpu)) {
> - if (!lowest)
> - lowest = vcpu;
> - else if (kvm_apic_compare_prio(vcpu, lowest) < 0)
> - lowest = vcpu;
> + if (!kvm_vector_hashing_enabled()) {
> + if (!lowest)
> + lowest = vcpu;
> + else if (kvm_apic_compare_prio(vcpu, lowest) <
> 0)
> + lowest = vcpu;
> + } else {
> + __set_bit(vcpu->vcpu_id, dest_vcpu_bitmap);
> + dest_vcpus++;
> + }
> }
> }
>
> + if (dest_vcpus != 0) {
> + idx = kvm_vector_2_index(irq->vector, dest_vcpus,
> + dest_vcpu_bitmap, KVM_MAX_VCPUS);
> +
> + lowest = kvm_get_vcpu(kvm, idx - 1);
This is going to fail with sparse topologies (e.g. 3 cores per socket).
vcpu_id = initial APIC ID and kvm_get_vcpu() uses a compressed array
that has kvm->online_vcpus elements, so we could overflow.
The 'i' in kvm_for_each_vcpu() could be used for the bitmap.
(kvm_get_vcpu_by_id() instead of kvm_get_vcpu() is slightly worse.)
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> @@ -678,6 +678,22 @@ bool kvm_apic_match_dest(struct kvm_vcpu *vcpu, struct
> kvm_lapic *source,
> bool kvm_irq_delivery_to_apic_fast(struct kvm *kvm, struct kvm_lapic *src,
> struct kvm_lapic_irq *irq, int *r, unsigned long *dest_map)
> {
> @@ -731,17 +747,38 @@ bool kvm_irq_delivery_to_apic_fast(struct kvm *kvm,
> struct kvm_lapic *src,
> + if (!kvm_vector_hashing_enabled()) {
| [...]
> + } else {
> + int idx = 0;
> + unsigned int dest_vcpus = 0;
Now that we don't need to check for present/enabled LAPICs, I think it
would be better to solve this by assuming that all selected LAPICs are
enabled, so the n-th target is decided only based on vector and
destination.
> + for_each_set_bit(i, &bitmap, 16) {
> + if (!dst[i] &&
> !kvm_lapic_enabled(dst[i]->vcpu)) {
> + __clear_bit(i, &bitmap);
> + continue;
> + }
> + }
=> we could skip this loop.
> +
> + dest_vcpus = hweight16(bitmap);
> +
> + if (dest_vcpus != 0) {
> + idx = kvm_vector_2_index(irq->vector,
> + dest_vcpus, &bitmap, 16);
> +
> + bitmap = 0;
> + __set_bit(idx-1, &bitmap);
And set just this bit.
The drawback is that buggy software that included hardware disabled
APICs to lowest priority destinations could stop working ...
Do you think it's too risky?
> + }
> }
(This is basically the same as converting the message to a fixed delivery
to n-th bit beforehand, so it might be reasonable to to apply something
similar to simplify the slow path as well. Mixed flat/cluster/x2APIC
mode makes me suspect that it won't be reasonable.)
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