Re: [Xen-devel] [PATCH RFC v5 7/8] pvqspinlock, x86: Add qspinlock para-virtualization support

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

Il 27/02/2014 15:18, David Vrabel ha scritto:
> On 27/02/14 13:11, Paolo Bonzini wrote:
> > Il 27/02/2014 13:11, David Vrabel ha scritto:
> > > > > This patch adds para-virtualization support to the queue spinlock code
> > > > > by enabling the queue head to kick the lock holder CPU, if known,
> > > > > in when the lock isn't released for a certain amount of time. It
> > > > > also enables the mutual monitoring of the queue head CPU and the
> > > > > following node CPU in the queue to make sure that their CPUs will
> > > > > stay scheduled in.
> > > I'm not really understanding how this is supposed to work.  There
> > > appears to be an assumption that a guest can keep one of its VCPUs
> > > running by repeatedly kicking it?  This is not possible under Xen and I
> > > doubt it's possible under KVM or any other hypervisor.
> > KVM allows any VCPU to wake up a currently halted VCPU of its choice,
> > see Documentation/virtual/kvm/hypercalls.txt.
> But neither of the VCPUs being kicked here are halted -- they're either
> running or runnable (descheduled by the hypervisor).
/me actually looks at Waiman's code...

Right, this is really different from pvticketlocks, where the *unlock* 
primitive wakes up a sleeping VCPU.  It is more similar to PLE 
(pause-loop exiting).
Paolo

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