Re: [Xen-devel] [PATCH v2] xen/arm: introduce vwfi parameter

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 22/02/17 17:29, Stefano Stabellini wrote:
> Introduce new Xen command line parameter called "vwfi", which stands for
> virtual wfi. The default is "trap": Xen traps the guest wfi instruction
> and calls vcpu_block on the guest vcpu. The behavior can be changed
> setting vwfi to "native", in that case Xen doesn't trap the instruction
> at all, running it in guest context.
>
> The result is strong reduction in irq latency (from 5000ns to 2000ns,
> measured using https://github.com/edgarigl/tbm, the physical timer, and
> 1 pcpu dedicated to 1 vcpu). The downside is that the scheduler thinks
> that the guest is busy when actually is sleeping, leading to suboptimal
> scheduling decisions.
This patch is reducing WFI latency and I think we should have similar 
option for WFE.
WFE will be used for instance when a processor is trying to acquire a 
lock already taken and wait until an event is received. So you want to a 
good latency here too.
We should probably have a single option handling the both. I am not sure 
whether it is worth to allow native for one but not the other.
Cheers,

--
Julien Grall

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