Re: [PATCH 3/4] paravirt: add virt_spin_lock pvops function

[Peter Zijlstra]

On Wed, Sep 06, 2017 at 08:44:09AM -0400, Waiman Long wrote:
> On 09/06/2017 03:08 AM, Peter Zijlstra wrote:
> > Guys, please trim email.
> >
> > On Tue, Sep 05, 2017 at 10:31:46AM -0400, Waiman Long wrote:
> >> For clarification, I was actually asking if you consider just adding one
> >> more jump label to skip it for Xen/KVM instead of making
> >> virt_spin_lock() a pv-op.
> > I don't understand. What performance are you worried about. Native will
> > now do: "xor rax,rax; jnz some_cold_label" that's fairly trival code.
> 
> It is not native that I am talking about. I am worry about VM with
> non-Xen/KVM hypervisor where virt_spin_lock() will actually be called.
> Now that function will become a callee-saved function call instead of
> being inlined into the native slowpath function.

But only if we actually end up using the test-and-set thing, because if
you have paravirt we end up using that.

And the test-and-set thing sucks anyway. But yes, you're right, that
case gets worse.