Re: [Qemu-devel] [RFC v2] Add support for KVM_CAP_SPLIT_IRQCHIP

[Paolo Bonzini]

On 02/11/2015 22:19, Matt Gingell wrote:
> Hi Jan, 
> 
> Would you be able to look this over? I’d like to get this into shape to 
> commit, 
> either now or once the corresponding kernel patch goes in.
> 
> Thanks,
> Matt
> 
> Add support for KVM_CAP_SPLIT_IRQCHIP
> 
> Adds a new alternative 'split' to the -machine kernel-irqchip option.
> When split mode is specified:
> 
>      1.) KVM_CAP_SPLIT_IRQCHIP is enabled.
> 
>      2.) The PIC, PIT, and IOAPIC are implemented in userspace while the
>      LAPIC is implemented by KVM.
> 
>      3.) The software IOAPIC delivers interrupts to the KVM LAPIC via
>      kvm_set_irq. Interrupt delivery is configured via the MSI routing
>      table, for which routes are reserved in target-i386/kvm.c then
>      configured in hw/intc/ioapic.c
> 
>      4.) KVM delivers IOAPIC EOIs via a new exit KVM_EXIT_IOAPIC_EOI,
>      which is handled in target-i386/kvm.c and relayed to the software
>      IOAPIC via ioapic_eoi_broadcast.

I had looked at v1, and there was nothing surprising.  I was travelling
and couldn't comment immediately.

I only have a couple of notes:

1) the kvm_*_in_kernel() rename should be a separate patch;

2) this:

+    if (machine_kernel_irqchip_split(machine)) {
+        ret = kvm_vm_enable_cap(s, KVM_CAP_SPLIT_IRQCHIP, 0, 24);
+        if (ret) {
+            error_report("Could not enable split irqchip mode: %s\n",
+                         strerror(-ret));

and "bool kvm_split_irqchip" should go in target-i386/kvm.c, using the
existing kvm_arch_irqchip_create hook;

3) other arches than i386 should reject kernel_irqchip=split.

More importantly, I would like to understand what is the kernel issue
that you are getting.  It should be resolved before 4.4 goes out in two
months, of course.

Paolo