Re: VT-x and Performance counter interrupt in KVM mode

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Shobha Ranganathan wrote:
> I am trying to capture in vmx.c the hardware
> performance counter(PMU) interrupt of a i386 Linux
> kernel running with perfmon on a Core 2 Duo machine
> running with kvm-15. host is running kvm with VT-x in
> x86-64 mode.
>
> The PMU interrupt is programmed in the APIC LVT entry
> (set to 0xee)by the guest OS. 

On stock kvm, the guest os programs a virtual apic that lives in qemu, 
not the real apic, so it would never cause any interrupt.  Are you 
running with a modified kvm that allows the guest to touch the real apic?


> Similarly, an IDT entry
> connects the interrupt vector to the interrupt
> handler.
> I am not able to catch, in kvm, the PMU interrupt
> happening in VMX non-root mode. It does not seem to
> appear in the VM-exit interruption information nor in
> the IDT-vectoring information. It does not seem to
> be caught by any of the exit handlers yet the host PMU
> interrupt handler catches it which is not what we
> want.
>
> Any idea on what is going on with this interrupt?
>   

It looks completely normal, assuming the host also programmed the timer 
to the same vector.  Look in qemu/hw/apic.c to find your missing interrupt.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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