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

From: "wangyanan (Y)" <wangyanan55@huawei.com>
To: "Andrew Jones" <drjones@redhat.com>,
	"Daniel P. Berrangé" <berrange@redhat.com>
Cc: Barry Song <song.bao.hua@hisilicon.com>,
	Peter Maydell <peter.maydell@linaro.org>,
	"Michael S . Tsirkin" <mst@redhat.com>,
	wanghaibin.wang@huawei.com, qemu-devel@nongnu.org,
	yangyicong@huawei.com, Shannon Zhao <shannon.zhaosl@gmail.com>,
	qemu-arm@nongnu.org, Alistair Francis <alistair.francis@wdc.com>,
	prime.zeng@hisilicon.com, Paolo Bonzini <pbonzini@redhat.com>,
	yuzenghui@huawei.com, Igor Mammedov <imammedo@redhat.com>,
	zhukeqian1@huawei.com, David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [RFC PATCH v4 0/7] hw/arm/virt: Introduce cpu topology support
Date: Tue, 22 Jun 2021 20:31:22 +0800	[thread overview]
Message-ID: <bc47a66a-b1ff-939c-32a2-94c90efd0caf@huawei.com> (raw)
In-Reply-To: <20210622114634.crjqusw6x6oj4j6v@gator>

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
>
> .