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.
INSTR
CPUID 1394
RDMSR 1550
MMIO
APIC 2609
IOAPIC 2800
HPET 9426
PIO
PIC 1804
UART 8011
