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@xxxxxx> 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@xxxxxx> 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@xxxxxx> 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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