On 2015-02-15 16:59, Christoffer Dall wrote: > 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. I'm running vexpress, but I only tried legacy -no-kvm-irqchip so far which was refused. -machine vexpress-a15,kernel_irqchip=off has an effect: host practically locks up, dmesg - when I'm still able to start on a different console - gives endless "Unexpected interrupt 19 on vcpu ecd39670". Well, a different smell, but still very fishy. Jan
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