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

[Juergen Gross]

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?

In case you agree I can setup a patch...


Juergen