Re: [Xen-devel] [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Jul 10, 2013 at 01:47:17PM +0300, Gleb Natapov wrote:
> On Wed, Jul 10, 2013 at 12:40:47PM +0200, Peter Zijlstra wrote:
> > On Wed, Jul 10, 2013 at 01:33:25PM +0300, Gleb Natapov wrote:
> > 
> > Here's an idea, trim the damn email ;-) -- not only directed at gleb.
> > 
> Good idea.
> 
> > > > Ingo, Gleb,
> > > > 
> > > > From the results perspective, Andrew Theurer, Vinod's test results are
> > > > pro-pvspinlock.
> > > > Could you please help me to know what will make it a mergeable
> > > > candidate?.
> > > > 
> > > I need to spend more time reviewing it :) The problem with PV interfaces
> > > is that they are easy to add but hard to get rid of if better solution
> > > (HW or otherwise) appears.
> > 
> > How so? Just make sure the registration for the PV interface is optional; 
> > that
> > is, allow it to fail. A guest that fails the PV setup will either have to 
> > try
> > another PV interface or fall back to 'native'.
> > 
> We have to carry PV around for live migration purposes. PV interface
> cannot disappear under a running guest.

Why can't it? This is the same as handling say XSAVE operations. Some hosts
might have it - some might not. It is the job of the toolstack to make sure
to not migrate to the hosts which don't have it. Or bound the guest to the
lowest interface (so don't enable the PV interface if the other hosts in the
cluster can't support this flag)?

> 
> > > > I agree that Jiannan's Preemptable Lock idea is promising and we could
> > > > evaluate that  approach, and make the best one get into kernel and also
> > > > will carry on discussion with Jiannan to improve that patch.
> > > That would be great. The work is stalled from what I can tell.
> > 
> > I absolutely hated that stuff because it wrecked the native code.
> Yes, the idea was to hide it from native code behind PV hooks.
> 
> --
>                       Gleb.

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