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

[wangyanan (Y)]

On 2021/6/22 19:46, Andrew Jones wrote:
> 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?
Yes, we indeed discuss a new "strict" parameter but not a "expose" in v2 
[1] of this series.
[1] 
https://patchwork.kernel.org/project/qemu-devel/patch/20210413080745.33004-6-wangyanan55@huawei.com/

And in the discussion, we hoped things would work like below with 
"strict" parameter:
Users who want to describe cpu topology should provide cmdline like

-smp strict=on,cpus=4,sockets=2,cores=2,threads=1

and in this case we require an more accurate -smp configuration and
then generate the cpu topology description through ACPI/DT.

While without a strict description, no cpu topology description would
be generated, so they get nothing through ACPI/DT.

It seems to me that the "strict" parameter actually serves as a knob to
turn on/off the exposure of topology, and this is the reason I changed
the name.

Thanks,
Yanan
.
> Thanks,
> drew
>
> .