Re: [RFC PATCH v2 2/4] hw/arm/virt: Parse -smp cluster parameter in virt_smp_parse

["wangyanan (Y)" <wangyanan55@huawei.com>]

Hi Drew,

On 2021/4/30 18:49, Andrew Jones wrote:
> On Fri, Apr 30, 2021 at 05:33:42PM +0800, wangyanan (Y) wrote:
>> On 2021/4/30 15:01, Andrew Jones wrote:
>>> On Fri, Apr 30, 2021 at 08:41:25AM +0200, Andrew Jones wrote:
>>>> On Fri, Apr 30, 2021 at 01:09:00PM +0800, wangyanan (Y) wrote:
>>>>> But I think the requirement for ARM "if even one parameter other than cpus
>>>>> or maxcpus
>>>>> is provided then all parameters must be provided" will be better. This can
>>>>> ensure the
>>>>> whole accurate users-specified topology. As you mentioned, if anybody who
>>>>> bothers
>>>>> to specify one, why not also specify the others.
>>>>>
>>>>> I can add the requirement for ARM in the documentation, and also check the
>>>>> parameters
>>>>> in virt_smp_parse. Will this be fine?
>>>> We sort of have to support command lines that are missing 'maxcpus' and
>>>> 'clusters', unless we work together with libvirt to make the change.
>>>> Currently libvirt will generate '-smp 16,sockets=16,cores=1,threads=1'
>>>> from '<vcpu placement='static'>16</vcpu>'. That's sufficient for our
>>>> stricter, but not completely strict requirements. And, I still think
>>>> 'threads' could be optional, because there's a good chance the user
>>>> doesn't want to describe them, so a default of 1 is good enough. Also,
>>>> given maxcpus, but not cpus, it's pretty obvious that cpus should equal
>>>> maxcpus.
>>>>
>>> We also still need just 'cpus' or just 'maxcpus' to work, since that
>>> already works now. So, at least these should work
>>>
>>>    -smp N
>>>    -smp maxcpus=N
>>>    -smp N,maxcpus=M
>>>    -smp N,sockets=N,cores=1,threads=1
>>>    -smp N,maxcpus=M,sockets=M,cores=1,threads=1
>>>
>>> since they work today, even though no topology is described.
>> Yes. I forgot this point that we should consider the compat.
>>> If we want to
>>> describe a topology for the first three, then we'll have to pick one,
>>> which brings us back to the sockets over cores stuff. Or, we could choose
>>> to just not generate topology descriptions when none is provided.
>> I find that both preferring cores over sockets and not generating topology
>> descriptions
>> when none is provided can't solve everything. We can only ensure guest will
>> get one
>> socket with multiple cores with qemu cmdline "-smp N".
>>
>> But if we specify N cpus without any other parameters in libvirt xml, a qemu
>> cmdline like
>> "-smp N, sockets=N, cores=1,threads=1" will be generated, and as a result
>> guest will get
>> N sockets. In this case, we still can't avoid the silent change.
>>
>> So maybe we should just prefer sockets over cores like the general
>> smp_parse() and libvirt,
>> and let users use "-smp N, sockets=1, cores=N,threads=1" to get what they
>> want if they
>> use version 6.0 or later.
> Rather than doing the preference of sockets over cores thing, let's
> require that sockets, cores, and one of maxcpus or cpus are provided,
> as we discussed in the other mail.
Ok, I see.

Thanks for these discussions about parsing of -smp command line. :)
Now it seems that a separate virt_smp_parse() function for ARM is still 
necessary,
I will implement it in v3 of the other "Introduce cpu topology support" 
series.

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