On 13/05/2015 08:12, Jan Kiszka wrote:
>> +void kvm_scan_ioapic_routes(struct kvm_vcpu *vcpu, u64 *eoi_exit_bitmap)
>> +{
>> + struct kvm *kvm = vcpu->kvm;
>> + struct kvm_kernel_irq_routing_entry *entry;
>> + struct kvm_irq_routing_table *table;
>> + u32 i, nr_rt_entries;
>> +
>> + mutex_lock(&kvm->irq_lock);
This only needs irq_srcu protection, not irq_lock, so the lookup cost
becomes much smaller (all CPUs can proceed in parallel).
You would need to put an smp_mb here, to ensure that irq_routing is read
after KVM_SCAN_IOAPIC is cleared. You can introduce
smb_mb__after_srcu_read_lock in order to elide it.
The matching memory barrier would be a smp_mb__before_atomic in
kvm_make_scan_ioapic_request.
>> + table = kvm->irq_routing;
>> + nr_rt_entries = min_t(u32, table->nr_rt_entries, IOAPIC_NUM_PINS);
>> + for (i = 0; i < nr_rt_entries; ++i) {
>> + hlist_for_each_entry(entry, &table->map[i], link) {
>> + u32 dest_id, dest_mode;
>> +
>> + if (entry->type != KVM_IRQ_ROUTING_MSI)
>> + continue;
>> + dest_id = (entry->msi.address_lo >> 12) & 0xff;
>> + dest_mode = (entry->msi.address_lo >> 2) & 0x1;
>> + if (kvm_apic_match_dest(vcpu, NULL, 0, dest_id,
>> + dest_mode)) {
>> + u32 vector = entry->msi.data & 0xff;
>> +
>> + __set_bit(vector,
>> + (unsigned long *) eoi_exit_bitmap);
>> + }
>> + }
>> + }
>> + mutex_unlock(&kvm->irq_lock);
>> +}
>>
>
> This looks a bit frightening regarding the lookup costs. Do we really
> have to run through the complete routing table to find the needed
> information? There can be way more "real" MSI entries than IOAPIC pins.
It does at most IOAPIC_NUM_PINS iterations however.
> There can even be multiple IOAPICs (thanks to your patches overcoming
> the single in-kernel instance).
With multiple IOAPICs you have more than 24 GSIs per IOAPIC. That means
that the above loop is broken for multiple IOAPICs.
But perhaps when enabling KVM_SPLIT_IRQCHIP we can use args[0] to pass
the number of IOAPIC routes that will cause EOI exits?
Paolo
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