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

[Alexander Graf <agraf@suse.de>]

On 02.06.14 09:49, Eric Auger wrote:
> This patch aims at optimizing IRQ handling using irqfd framework.
> It brings significant performance improvement over "traditional" IRQ
> handling introduced in :
> "vfio: Add initial IRQ support in platform device".
>
> This new IRQ handling method depends on kernel KVM irqfd/GSI routing
> capability.
>
> The IRQ handling method can be dynamically chosen (default is irqfd,
> if kernel supports it obviously).  For example to disable irqfd
> handling, use:
>
> -device vfio-platform,vfio_device="fff51000.ethernet",\
> compat="calxeda/hb-xgmac",mmap-timeout-ms=110,irqfd=false\
>
> Performances are improved for the following reasons:
> - eventfds signalled by the VFIO platform driver are handled on
>    kernel side by the KVM irqfd framework.
> - the end of interrupt(EOI) is trapped at GIC level and not at MMIO
>    region level. As a reminder, in traditional IRQ handling QEMU
>    assumed the first guest access to a device MMIO region after IRQ
>    hit was the IRQ status register reset. This trap was approximate
>    and obliged to swap to slow path after IRQ hit. A mmap timer
>    mechanism enabled to swap back to fast path after the mmap period
>    introducing extra complexity. Now GIC detects the completion of
>    the virtual IRQ and signals a resampler eventfd on maintenance
>    IRQ. The corresponding handler re-enables the physical IRQ.

Ah, so if you're using irqfd you do unmask the interrupt on EOI. Why not 
without irqfd? And if the answer is "because it's too difficult" - why 
support VFIO without irqfd at all then?


Alex