Re: [PATCH] KVM: drop the kvm_has_noapic_vcpu optimization

[Sean Christopherson <seanjc@google.com>]

On Fri, Nov 01, 2024, Bernhard Kauer wrote:
> On Thu, Oct 31, 2024 at 04:29:52PM -0700, Sean Christopherson wrote:
> > > > Unless your VM doesn't need a timer and doesn't need interrupts of
> > > > any kind, emulating the local APIC in userspace is going to be much
> > > > less performant.
> > > 
> > > Do you have any performance numbers?
> > 
> > Heh, nope.  I actually tried to grab some, mostly out of curiosity again, but
> > recent (last few years) versions of QEMU don't even support a userspace APIC.
> > 
> > A single EOI is a great example though.  On a remotely modern CPU, an in-kernel
> > APIC allows KVM to enable hardware acceleration so that the EOI is virtualized by
> > hardware, i.e. doesn't take a VM-Exit and so the latency is basically the same as
> > a native EOI (tens of cycles, maybe less).
> > 
> > With a userspace APIC, the roundtrip to userspace to emulate the EOI is measured
> > in tens of thousands of cycles.  IIRC, last I played around with userspace exits
> > the average turnaround time was ~50k cycles.
> 
> 
> That sound a lot so I did some quick benchmarking.  An exit is around 1400
> TSC cycles on my AMD laptop, instruction emulation takes 1200 and going
> to user-level needs at least 6200.  Not terribly slow but still room for
> optimizations.

Ah, I suspect my recollection of ~50k cycles is from measuring all exits to
userspace, i.e. included the reaaaaly slow paths.