Re: [PATCH v8 3/9] KVM: arm/arm64: Don't cache the timer IRQ level

Marc Zyngier
2017-12-13T19:38:44Z

On Wed, 13 Dec 2017 10:45:56 +0000,
Christoffer Dall wrote:
>
> The timer was modeled after a strict idea of modelling an interrupt line
> level in software, meaning that only transitions in the level needed to
> be reported to the VGIC. This works well for the timer, because the
> arch timer code is in complete control of the device and can track the
> transitions of the line.
>
> However, as we are about to support using the HW bit in the VGIC not
> just for the timer, but also for VFIO which cannot track transitions of
> the interrupt line, we have to decide on an interface for level
> triggered mapped interrupts to the GIC, which both the timer and VFIO
> can use.
>
> VFIO only sees an asserting transition of the physical interrupt line,
> and tells the VGIC when that happens. That means that part of the
> interrupt flow is offloaded to the hardware.
>
> To use the same interface for VFIO devices and the timer, we therefore
> have to change the timer (we cannot change VFIO because it doesn't know
> the details of the device it is assigning to a VM).
>
> Luckily, changing the timer is simple, we just need to stop 'caching'
> the line level, but instead let the VGIC know the state of the timer
> every time there is a potential change in the line level, and when the
> line level should be asserted from the timer ISR. The VGIC can ignore
> extra notifications using its validate mechanism.
>
> Reviewed-by: Andre Przywara <andre.przywara@arm.com>
> Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>

Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>

M.