Re: [PATCH v2] KVM: x86: Drop the kvm_has_noapic_vcpu optimization

Bernhard Kauer
2024-12-10T08:16:46Z

On Mon, Dec 09, 2024 at 05:40:48PM -0800, Sean Christopherson wrote:
> On Mon, Dec 09, 2024, Sean Christopherson wrote:
> > On Mon, Oct 21, 2024, Bernhard Kauer wrote:
> > > It used a static key to avoid loading the lapic pointer from
> > > the vcpu->arch structure. However, in the common case the load
> > > is from a hot cacheline and the CPU should be able to perfectly
> > > predict it. Thus there is no upside of this premature optimization.
> > >
> > > The downside is that code patching including an IPI to all CPUs
> > > is required whenever the first VM without an lapic is created or
> > > the last is destroyed.
> > >
> > I'm on the fence, slightly leaning towards removing all three of these static keys.

Thanks for continuing this work.

> > With a single vCPU pinned to a single pCPU, the average latency for a CPUID exit
> > goes from 1018 => 1027 cycles, plus or minus a few. With 8 vCPUs, no pinning
> > (mostly laziness), the average latency goes from 1034 => 1053.

Are these kind of benchmarks tracked somewhere automatically? With it one
could systematically optimize for faster exits.

> > On the other hand, we lose gobs and gobs of cycles with far less thought. E.g.
> > with mitigations on, the latency for a single vCPU jumps all the way to 1600+ cycles.

In the end it is a tradeoff to be made. The cost for switching between the
modes is more than a hundred microsecond unexpected latency. On the other
hande one saves 1-2% per exit but has a larger code-base.