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

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).


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