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

From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH] kvm: Move kvm_allows_irq0_override() to target-i386
Date: Sat, 21 Jul 2012 09:54:59 +0100

On 21 July 2012 07:57, Jan Kiszka <address@hidden> wrote:
> On 2012-07-20 21:14, Peter Maydell wrote:
>> I'm sure this isn't the only x86ism in the KVM generic source
>> files. However the thing I'm specifically trying to do is
>> nuke all the uses of kvm_irqchip_in_kernel() in common code,
> No, "irqchip in kernel" is supposed to be a generic concept. We will
> also have it on Power. Not sure what your plans are for ARM, maybe it
> will always be true there.

I agree that "irqchip in kernel?" is generic (though as you'll see
below there's disagreement about what that ought to mean or imply).
"irq0_override" though seems to me to be absolutely x86 specific.

> That said, maybe there is room for discussion about what it means for
> the general KVM code and its users if the irqchip is in the kernel. Two
> things that should be common for every arch:
>  - VCPU idle management is done inside the kernel

The trouble is that at the moment QEMU assumes that "is the
irqchip in kernel?" == "is VCPU idle management in kernel", for
instance. For ARM, VCPU idle management is in kernel whether
we're using the kernel's model of the VGIC or not. Alex tells
me PPC is the same way. It's only x86 that has tied these two
concepts together.

The reason I want to get rid of common-code uses of kvm_irqchip_in_kernel()
is because I think they're all similar to this -- the common code is
using the check as a proxy for something else, and it should be directly
asking about that something else. The only bits of code that should
care about "is the irqchip in kernel?" are:
 * target-specific device/machine setup code which needs to know
   which apic/etc to instantiate
 * target-specific x86 code which has this weird synchronous IRQ
   delivery model for irqchip-not-in-kernel
(Obviously I might have missed something, I'm flailing around
trying to understand this code :-))

>  - in-kernel KVM helpers like vhost or VFIO can inject IRQs directly
> The latter point implies that irqfd is available and that interrupt
> routes from virtual IRQs (*) (like the one associated with an irqfd) to
> the in-kernel IRQ controller have to be established. That's pretty generic.

But you can perfectly well have an in-kernel-irqchip that doesn't
support irqfd -- it just means that interrupts from devices have
to come in via the ioctls same as anything else. Some in-kernel
helpers obviously would depend on the existence and use of a full
featured in-kernel irqchip (on ARM you don't get the in kernel timer
unless you have in kernel VGIC), but I don't see why the virtio code
should be asking "is there an in kernel irqchip?" rather than "can
I do irqfd routing?" or whatever the question is it actually wants
to ask.  (In fact the virtio code probably needs to do something
more complex anyway: you could perfectly well have a system where
there is a full-featured irqchip in the kernel but the virtio
device is on the "wrong" side of a second interrupt controller
which is not in-kernel. So the actual question it needs to ask
is "does the interrupt wiring in this specific machine model mean
I can get and use an irqfd from where I am to the main CPU
interrupt controller?" or something similar.)

>> as part of trying to untangle "what irqchip model do you want"
>> from other aspects of the interrupt handling model.
>> Other than this one, there are uses in cpus.c and hw/virtio-pci.c,
>> but I haven't figured out yet exactly what those bits of code
>> are trying to do...
> See above for the two reasons.

Thanks for the explanations.

-- PMM

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