On 05/09/17 10:28, Peter Zijlstra wrote: > On Tue, Sep 05, 2017 at 10:14:21AM +0200, Juergen Gross wrote: >> On 05/09/17 10:10, Peter Zijlstra wrote: >>> 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. >>> >> >> Aah, now I've got it. >> >> So maybe we should add virt_spin_lock() to pv_lock_ops? This way e.g. >> xen_nopvspin could tweak just the virt_spin_lock() case by letting it >> return false all the time? > > Hmm, that might work. Could we somehow nop that call when > !X86_FEATURE_HYPERVISOR?, that saves native from having to do the call > and would be a win for everyone. So in fact we want a "always false" shortcut for bare metal and for any virtualization environment selecting bare metal behavior. I'll have a try. Juergen
