Re: [PATCH] kvm-ia64 irq assignment 1/2 kernel

[Avi Kivity <avi@qumranet.com>]

Xu, Anthony wrote:
> In kvm-ia64, we use the same guest firmware (GFW)as in Xen, 
> GFW uses PRT to present PCI interrupt routing, all PCI devices'
> interrupt pins
> connect to IOAPIC, which doesn't match with kvm-ia64 Qemu PCI interrupt
> routing.
>
> This patch modify Qemu PCI interupt routing code to match with GFW, 
> Then PCI devices in qemu can work in kvm-ia64, for exmaple, NIC
>
>
> Signed-off-by: Anthony Xu < anthony.xu@intel.com >
>
>
> diff --git a/virt/kvm/ioapic.c b/virt/kvm/ioapic.c
> index 99a1736..80c116c 100644
> --- a/virt/kvm/ioapic.c
> +++ b/virt/kvm/ioapic.c
> @@ -272,7 +272,11 @@ void kvm_ioapic_set_irq(struct kvm_ioapic *ioapic,
> int irq, int level)
>
>         if (irq >= 0 && irq < IOAPIC_NUM_PINS) {
>                 entry = ioapic->redirtbl[irq];
> -               level ^= entry.fields.polarity;
> +// polarity of all devices in qemu is active high
> +//  regardless of ioapic setting
> +
> +//             level ^= entry.fields.polarity;
> +
>   

There are two errors in this patch:

- kvm is not there just for qemu; it should be possible to use kvm with 
some other userspace, which would assume that kvm correctly emulates 
ioapic polarity
- you are modifying shared code and so affect x86 as well

I suggest modifying the firmware to report the interrupts as active 
high.  Since Xen does not emulate polarity, the change will not affect 
it and the firmware can continue to be shared.  I'd also recommend 
fixing Xen to emulate the polarity correctly, if possible.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.