Re: [Qemu-devel] [PATCH] kvm: Move kvm_allows_irq0_override() to target-i386

[Peter Maydell]

On 23 July 2012 14:09, Avi Kivity <address@hidden> wrote:
> On 07/23/2012 03:58 PM, Peter Maydell wrote:
>> So should we be using something other than KVM_CREATE_IRQCHIP to
>> ask the kernel to create a GIC model for us (and leave KVM_CREATE_IRQCHIP
>> as a dummy "always succeed" ioctl)?
>
> Some time ago I suggested using the parameter to KVM_CREATE_IRQCHIP to
> select the "irqchip type".

That seems reasonable, although we have an awkward ordering issue
in KVM as it stands: KVM_CREATE_IRQCHIP needs to be called before
we start creating VCPU threads (at the moment it is done via kvm_init)
but we don't know what kind of irqchip we want to create until the
machine model code actually creates the irqchip device, which generally
happens after the CPU object is created (and VCPU threads are created
at that point). We could fix that by moving creation of the actual
VCPU thread to vl.c after the model has been initialized.

>> I'm not sure ARM has any equivalent to the local APIC -- the GIC
>> deals with everything and we don't have any equivalent division
>> of labour to the x86 LAPIC-IOAPIC one.
>
> It's probably a tiny part of the core with no name.  The point is that
> the x86<->lapic interface is synchronous and bidirectional, while the
> lapic<->ioapic interface is asynchronous (it is still bidirectional, but
> not in a stop-the-vcpu way).  I assume the ARM<->GIC interface is
> unidirectional?

Well, strictly speaking the ARM<->GIC interface is implementation
defined, but in practice you can think of it as "the GIC controls
the IRQ and FIQ input lines to each core and uses them to signal
that an interrupt is present". There's no need for anything to
be signalled back in the other direction: the GIC will just continue
to hold IRQ asserted until the interrupt handler code writes the
relevant GIC register to indicate that the interrupt has been
handled.

(On a core with the virtualization extensions there are also
signals for the GIC to raise a virtual IRQ or FIQ, but we can
ignore those for KVM because we don't and can't provide the
virtualization extensions to a guest.)

-- PMM