Re: [Qemu-devel] errno 13, fopen qemu trace file.

[Stefan Hajnoczi <stefanha@gmail.com>]

On Thu, Aug 18, 2016 at 1:58 PM, Nir Levy <nirl@asocsnetworks.com> wrote:
> I have a progress in tracing qemu,
> I add the thread and tag done for each kvm_ioctl, kvm_vm_ioctl, kvm_vcpu_ioctl
> in purpose of investigating pure hypervisor activity and delays on host.
> the kvm type print only for convenience.
>
> for example:
>
> kvm_ioctl 3106435.230545 pid=11347 thread=11347 type=0xae03 arg=0x25
>
> kvm_ioctl_done 3106435.230546 pid=11347 thread=11347 type=0xae03 arg=0x25 diff=1 (KVM_CHECK_EXTENSION)
>
> kvm_vcpu_ioctl 3106435.253930 pid=11347 thread=11354 cpu_index=0x2 type=0x4008ae9c arg=0x56417e6cb4f0
>
> kvm_vcpu_ioctl_done 3106435.253931 pid=11347 thread=11354 cpu_index=0x2 type=0x4008ae9c arg=0x56417e6cb4f0 diff=1 (KVM_X86_SETUP_MCE)
>
> kvm_vm_ioctl 3106435.268896 pid=11347 thread=11347 type=0x4020ae46 arg=0x7ffed97cf9d0
>
> kvm_vm_ioctl_done 3106435.269082 pid=11347 thread=11347 type=0x4020ae46 arg=0x7ffed97cf9d0 diff=186 (KVM_SET_USER_MEMORY_REGION)
>
>
> I have notice KVM_RUN can take even seconds but that is probably low priority tasks,(io workers probably)

Please read Linux Documentation/virtual/kvm/api.txt to learn about the
ioctl calls.

KVM_RUN is *the* ioctl that executes guest code.  Unless a vcpu is
halted we should be inside KVM_RUN, so spending time inside this ioctl
is normal.

> but this 186micro second on the main qemu thread is suspicious and might cause application running over vm delays.

By "186micro second" you are referring to KVM_SET_USER_MEMORY_REGION
in the trace above.

Is this ioctl called in the critical path?  I doubt it since the
KVM_X86_SETUP_MCE ioctl in your trace happens during initialization
time from kvm_arch_init_vcpu() and is not in the critical path when
the guest is running.

Why worry about latencies that do not affect running guests?

Stefan