Re: [PATCH 1/3] arm/arm64: KVM: Fix arch timer behavior for disabled interrupts

[Christoffer Dall <christoffer.dall@linaro.org>]

On Sat, Oct 17, 2015 at 10:30:20PM +0200, Christoffer Dall wrote:
> We have an interesting issue when the guest disables the timer interrupt
> on the VGIC, which happens when turning VCPUs off using PSCI, for
> example.
> 
> The problem is that because the guest disables the virtual interrupt at
> the VGIC level, we never inject interrupts to the guest and therefore
> never mark the interrupt as active on the physical distributor.  The
> host also never takes the timer interrupt (we only use the timer device
> to trigger a guest exit and everything else is done in software), so the
> interrupt does not become active through normal means.
> 
> The result is that we keep entering the guest with a programmed timer
> that will always fire as soon as we context switch the hardware timer
> state and run the guest, preventing forward progress for the VCPU.
> 
> Since the active state on the physical distributor is really part of the
> timer logic, it is the job of our virtual arch timer driver to manage
> this state.
> 
> The timer->map->active boolean field indicates whether we have signalled
> this interrupt to the vgic and if that interrupt is still pending or
> active.  As long as that is the case, the hardware doesn't have to
> generate physical interrupts and therefore we mark the interrupt as
> active on the physical distributor.
> 
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Reported-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
> ---
Marc was worried about the performance implications of this fix on
Mustang given the potentially slow MMIO path to the GIC on that system,
so I ran some before and after applying this series:

BM	Hackbench	Kernbench	PbZip C		PbZip D
--	---------	---------	-------		-------
Before	17.94		51.66		17.69		10.59
After	18.14		51.62		17.82		10.62

The slight increase on hackbench is well within the variability (1.409
for the 8 runs behind these numbers) so I don't think this will be
noticable.  That said, there's room for optimizations here by only
touching the GIC on vcpu load/put and when the value changes, but I
think this is premature.

-Christoffer