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.
