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

[David Vrabel <david.vrabel@xxxxxxxxxx>]

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).

David

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