non-root mode and preemption

non-root mode and preemption

[Oscar Fernando Garcia]

Good Morning,

I have a simple question. In KVM code: /arch/x86/kvm/x86.c when the 
function vcpu_enter_guest is called, the run function ( 
kvm_x86_ops->run(vcpu)) is called with preemption and irqs disabled.
My question is, running in non-root mode with preemption and irqs 
disabled affects the latency of other processes or VPCUs running in the 
same CPU?


Thank you

Oscar

Re: non-root mode and preemption

[Paolo Bonzini]

On 13/06/2016 12:06, Oscar Fernando Garcia wrote:
> Good Morning,
> 
> I have a simple question. In KVM code: /arch/x86/kvm/x86.c when the
> function vcpu_enter_guest is called, the run function (
> kvm_x86_ops->run(vcpu)) is called with preemption and irqs disabled.
> My question is, running in non-root mode with preemption and irqs
> disabled affects the latency of other processes or VPCUs running in the
> same CPU?

Even though kvm_x86_ops->run itself is called with interrupts disabled,
interrupts are enabled while the guest runs.  Instead of being serviced
immediately by the processor, however, they cause a vmexit.  The
interrupt routine is then called during
kvm_x86_ops->handle_external_intr, and preemption is enabled again soon
after that in vcpu_enter_guest.

The extra latency introduced by KVM is very low, around 1000 clock
cycles or less than a microsecond.

Paolo