Re: arm: warning at virt/kvm/arm/vgic.c:1468

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

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On Sun, Feb 15, 2015 at 04:35:14PM +0100, Jan Kiszka wrote:
> On 2015-02-15 16:30, Marc Zyngier wrote:
> > On Sun, Feb 15 2015 at  3:07:50 pm GMT, Jan Kiszka <jan.kiszka@web.de> wrote:
> >> On 2015-02-15 15:59, Marc Zyngier wrote:
> >>> On Sun, Feb 15 2015 at  2:40:40 pm GMT, Jan Kiszka <jan.kiszka@web.de> wrote:
> >>>> On 2015-02-15 14:37, Marc Zyngier wrote:
> >>>>> On Sun, Feb 15 2015 at 8:53:30 am GMT, Jan Kiszka
> >>>>> <jan.kiszka@web.de> wrote:
> >>>>>> I'm now throwing trace_printk at my broken KVM. Already found out that I
> >>>>>> get ARM_EXCEPTION_IRQ every few 10 µs. Not seeing any irq_* traces,
> >>>>>> though. Weird.
> >>>>>
> >>>>> This very much looks like a screaming interrupt. At such a rate, no
> >>>>> wonder your VM make much progress. Can you find out which interrupt is
> >>>>> screaming like this? Looking at GICC_HPPIR should help, but you'll have
> >>>>> to map the CPU interface in HYP before being able to access it there.
> >>>>
> >>>> OK... let me figure this out. I had this suspect as well - the host gets
> >>>> a VM exit for each injected guest IRQ?
> >>>
> >>> Not exactly. There is a VM exit for each physical interrupt that fires
> >>> while the guest is running. Injecting an interrupt also causes a VM
> >>> exit, as we force the vcpu to reload its context.
> >>
> >> Ah, GICC != GICV - you are referring to host-side pending IRQs. Any
> >> hints on how to get access to that register would accelerate the
> >> analysis (ARM KVM code is still new to me).
> > 
> > Map the GICC region in HYP using create_hyp_io_mapping (see
> > vgic_v2_probe for an example of how we map GICH), and stash the read of
> > GICC_HPPIR before leaving HYP mode (and before saving the guest timer).
> 
> OK.
> 
> > 
> > BTW, when you look at /proc/interrupts on the host, don't you see an
> > interrupt that's a bit too eager to fire?
> 
> No - but that makes sense given that we do not enter any interrupt
> handler according to ftrace, thus there can't be any counter incrementation.
> 
> > 
> >>>> BTW, I also tried with in-kernel GIC disabled (in the kernel config),
> >>>> but I guess that's pointless. Linux seems to be stuck on a
> >>>> non-functional architectural timer then, right?
> >>>
> >>> Yes. Useful for bringup, but nothing more.
> >>
> >> Maybe we should perform a feature check and issue a warning from QEMU?
> > 
> > I'd assume this is already in place (but I almost never run QEMU, so I
> > could be wrong here).
> 
> Nope, QEMU starts up fine, just lets the guest starve while waiting for
> jiffies to increase.
> 

you should be able to turn the in-kernel irqchip off with a QEMU
command-line option and the that should prevent the kernel from adding
an arch-timer.  This would only work on the vexpress guest model though,
since the virt-board doesn't provide an emulated timer as a replacement.

-Christoffer

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