Re: [Qemu-devel] [RFC v3 05/10] vfio: Add initial IRQ support in platform device

[Eric Auger <eric.auger@linaro.org>]

On 06/25/2014 11:40 PM, Alex Williamson wrote:
> On Wed, 2014-06-25 at 23:28 +0200, Alexander Graf wrote:
>> On 02.06.14 09:49, Eric Auger wrote:
>>> This patch brings a first support for device IRQ assignment to a
>>> KVM guest. Code is inspired of PCI INTx code.
>>>
>>> General principle of IRQ handling:
>>>
>>> when a physical IRQ occurs, VFIO driver signals an eventfd that was
>>> registered by the QEMU VFIO platform device. The eventfd handler
>>> (vfio_intp_interrupt) injects the IRQ through QEMU/KVM and also
>>> disables MMIO region fast path (where MMIO regions are mapped as
>>> RAM). The purpose is to trap the IRQ status register guest reset.
>>> The physical interrupt is unmasked on the first read/write in any
>>> MMIO region. It was masked in the VFIO driver at the instant it
>>> signaled the eventfd.
>>
>> This doesn't sound like a very promising generic scheme to me. I can 
>> easily see devices requiring 2 or 3 or more accesses until they're 
>> pulling down the IRQ line. During that time interrupts will keep firing, 
>> queue up in the irqfd and get at us as spurious interrupts.
>>
>> Can't we handle it like PCI where we require devices to not share an 
>> interrupt line? Then we can just wait until the EOI in the interrupt 
>> controller.
Hi Alex,

Actually I transposed what was done for PCI INTx. For sure the virtual
IRQ completion instant is not precise but as Alex says latter on irqfd
should be used whenever possible for both precision aspects and
performance. Given the perf of this legacy solution for IRQ intensive IP
I would discourage to use that mode anyway. This is why I did not plan
to invest more on this mode.
> 
> QEMU's interrupt abstraction makes this really difficult and something
> that's not generally necessary outside of device assignment.  I spent a
> long time trying to figure out how we'd do it for PCI before I came up
> with this super generic hack that works surprisingly well.  Yes, we may
> get additional spurious interrupts, but from a host perspective they're
> rate limited by the guest poking hardware, so there's a feedback loop.
> Also note that assuming this is the same approach we take for PCI, this
> mode is only used for the non-KVM accelerated path.
Yes this is again exactly the same approach as for PCI. We now have full
irqfd + resamplefd support.

Best Regards

Eric
>  When we have a KVM
> irqchip that supports a resampling irqfd then we can get an eventfd
> signal back at the point when we should unmask the interrupt on the
> host.  Creating a cross-architecture QEMU interface to give you a
> callback when the architecture's notion of a resampling event occurs is
> not a trivial undertaking.  Thanks,
> 
> Alex
>