Re: [PATCH] KVM: drop the kvm_has_noapic_vcpu optimization

[Sean Christopherson <seanjc@google.com>]

On Fri, Oct 18, 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.

Do you happen to have performance numbers?  I've been itching for an excuse to
excise this code for a few years now, the only reason I haven't ripped it out is
because I didn't want to do so without numbers to back up my claim that it's a
premature optimization.

In other words, I agree with your analysis, but I don't want to yank out the code
without at least _some_ numbers to back up the claim, because then we're essentially
committing the same crime of optimizing without measuring.

> 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.

In practice, this almost never happens though.  Do you have a use case for
creating VMs without in-kernel local APICs?