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Re: [Xen-devel] [PROPOSAL] Event channel for SMP-VMs: per-vCPU or per-OS?

On Wed, Oct 30, 2013 at 03:40:25PM +0800, Luwei Cheng wrote:
> >
> > What I mean is that you will introduce latency / performance penalty
> > from locks to protect critical sections. Say, if several CPUs contents
> > for same event, overall performance might downgrade.
> I agree with you to some extent. But the question is: how frequently such
> "contention" will happen? As explained, upcall handler is called only when
> the processor switches from the hypervisor to the guest OS, and trapping
> into the hypervisor are mostly caused by things like hypercall, IPI, etc.

PV guest traps into hypervisor everytime it enables interrupt.

> The probability that multiple switches happen "exactly" at the same same,
> which I guess, is very small..

It's not about "exactly at the same time", it's about we need to ensure
the handler runs only once (takes effect only once).

> Thanks,
> Luwei

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