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

[Peter Zijlstra]

On Tue, Sep 05, 2017 at 09:35:40AM +0200, Juergen Gross wrote:
> > So the problem with qspinlock is that it will revert to a classic
> > test-and-set spinlock if you don't do paravirt but are running a HV.
> 
> In the Xen case we just use the bare metal settings when xen_nopvspin
> has been specified. So paravirt, but without modifying any pv_lock_ops
> functions.

See arch/x86/include/asm/qspinlock.h:virt_spin_lock(). Unless you clear
X86_FEATURE_HYPERVISOR you get a test-and-set spinlock.

And as the comment there says, this is a fallback for !paravirt enabled
hypervisors to avoid the worst of the lock holder preemption crud.

But this very much does not deal with the 1:1 case nicely.