Re: [PATCH resend] x86,kvm: Add a kernel parameter to disable PV spinlock

[Peter Zijlstra]

On Tue, Sep 05, 2017 at 08:57:16AM +0200, Oscar Salvador wrote:
> It may be that the original patch was just to keep consistency between Xen
> and KVM, and also only for testing purposes.
> But we find a case when a customer of ours is running some workloads with
> 1<->1 mapping between physical cores and virtual cores, and we realized that
> with the pv spinlocks disabled there is a 4-5% of performance gain.

There are very definite downsides to using a test-and-set spinlock.

A much better option would be one that forces the use of native
qspinlock in the 1:1 case. That means you have to fail both pv_enabled()
and virt_spin_lock().