Re: [RFC PATCH v4 0/7] hw/arm/virt: Introduce cpu topology support

[Andrew Jones]

On Tue, Jun 22, 2021 at 11:18:09AM +0100, Daniel P. Berrangé wrote:
> On Tue, Jun 22, 2021 at 05:34:06PM +0800, Yanan Wang wrote:
> > Hi,
> > 
> > This is v4 of the series [1] that I posted to introduce support for
> > generating cpu topology descriptions to guest. Comments are welcome!
> > 
> > Description:
> > Once the view of an accurate virtual cpu topology is provided to guest,
> > with a well-designed vCPU pinning to the pCPU we may get a huge benefit,
> > e.g., the scheduling performance improvement. See Dario Faggioli's
> > research and the related performance tests in [2] for reference. So here
> > we go, this patch series introduces cpu topology support for ARM platform.
> > 
> > In this series, instead of quietly enforcing the support for the latest
> > machine type, a new parameter "expose=on|off" in -smp command line is
> > introduced to leave QEMU users a choice to decide whether to enable the
> > feature or not. This will allow the feature to work on different machine
> > types and also ideally compat with already in-use -smp command lines.
> > Also we make much stricter requirement for the topology configuration
> > with "expose=on".
> 
> Seeing this 'expose=on' parameter feels to me like we're adding a
> "make-it-work=yes" parameter. IMHO this is just something that should
> be done by default for the current machine type version and beyond.
> I don't see the need for a parameter to turnthis on, especially since
> it is being made architecture specific.
>

I agree.

Yanan, we never discussed an "expose" parameter in the previous versions
of this series. We discussed a "strict" parameter though, which would
allow existing command lines to "work" using assumptions of what the user
meant and strict=on users to get what they mean or an error saying that
they asked for something that won't work or would require unreasonable
assumptions. Why was this changed to an "expose" parameter?

Thanks,
drew