[Xen-devel] Re: [PATCH 2/3] kvm hypervisor : Add hypercalls to support pv-ticketlock

[Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>]

On Wed, Jan 19, 2011 at 10:53:52AM -0800, Jeremy Fitzhardinge wrote:
> > I didn't really read the patch, and I totally forgot everything from
> > when I looked at the Xen series, but does the Xen/KVM hypercall
> > interface for this include the vcpu to await the kick from?
> >
> > My guess is not, since the ticket locks used don't know who the owner
> > is, which is of course, sad. There are FIFO spinlock implementations
> > that can do this though.. although I think they all have a bigger memory
> > footprint.
> 
> At least in the Xen code, a current owner isn't very useful, because we
> need the current owner to kick the *next* owner to life at release time,
> which we can't do without some structure recording which ticket belongs
> to which cpu.

If we had a yield-to [1] sort of interface _and_ information on which vcpu
owns a lock, then lock-spinners can yield-to the owning vcpu, while the
unlocking vcpu can yield-to the next-vcpu-in-waiting. The key here is not to
sleep when waiting for locks (as implemented by current patch-series, which can 
put other VMs at an advantage by giving them more time than they are entitled 
to) and also to ensure that lock-owner as well as the next-in-line lock-owner 
are not unduly made to wait for cpu. 

Is there a way we can dynamically expand the size of lock only upon contention
to include additional information like owning vcpu? Have the lock point to a
per-cpu area upon contention where additional details can be stored perhaps?

1. https://lkml.org/lkml/2011/1/14/44

- vatsa

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