Re: [RFC PATCH v2 5/6] hw/arm/virt-acpi-build: Add PPTT table

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

On 2021/5/19 16:15, Andrew Jones wrote:
> On Wed, May 19, 2021 at 07:54:37AM +0000, Salil Mehta wrote:
>>> From: wangyanan (Y)
>>> Sent: Wednesday, May 19, 2021 4:18 AM
>>>
>>>
>>> On 2021/5/19 3:22, Salil Mehta wrote:
>>>>> From: Andrew Jones [mailto:drjones@redhat.com]
>>>>> Sent: Tuesday, May 18, 2021 8:06 PM
>>>>> To: Salil Mehta <salil.mehta@huawei.com>
>>>>> Cc: wangyanan (Y) <wangyanan55@huawei.com>; Peter Maydell
>>>>> <peter.maydell@linaro.org>; Michael S . Tsirkin <mst@redhat.com>; Wanghaibin
>>>>> (D) <wanghaibin.wang@huawei.com>; qemu-devel@nongnu.org; Shannon Zhao
>>>>> <shannon.zhaosl@gmail.com>; qemu-arm@nongnu.org; Alistair Francis
>>>>> <alistair.francis@wdc.com>; Zengtao (B) <prime.zeng@hisilicon.com>;
>>>>> yangyicong <yangyicong@huawei.com>; yuzenghui <yuzenghui@huawei.com>; Igor
>>>>> Mammedov <imammedo@redhat.com>; zhukeqian <zhukeqian1@huawei.com>; lijiajie
>>> (H)
>>>>> <lijiajie11@huawei.com>; David Gibson <david@gibson.dropbear.id.au>;
>>> Linuxarm
>>>>> <linuxarm@huawei.com>; linuxarm@openeuler.org
>>>>> Subject: Re: [RFC PATCH v2 5/6] hw/arm/virt-acpi-build: Add PPTT table
>>>>>
>>>>> On Tue, May 18, 2021 at 06:34:08PM +0000, Salil Mehta wrote:
>>>>>>    Those benefits, when vcpu pinning is used, are the same benefits
>>>>>>> as for the host, which already use PPTT tables to describe topology, even
>>>>>>> though hot plug isn't supported.
>>>>>> yes sure, you mean pinning vcpus according to the cpu topology for performance?
>>>>> Yup
>>>> Already Agreed :)
>>>>
>>>>>>> Now, if you're saying we should only generate tables for smp.cpus, not
>>>>>> Correct. This is what I thought we must be doing even now
>>>>>>
>>>>>>> smp.maxcpus, because hot plug isn't supported anyway, then I see your
>>>>>>> point. But, it'd be better to require smp.cpus == smp.maxcpus in our
>>>>>>> smp_parse function to do that, which we've never done before, so we may
>>>>>>> have trouble supporting existing command lines.
>>>>>> I am trying to recall, if the vcpu Hotplug is not supported then can they
>>>>>> ever be different?
>>>>>>
>>>>>> cpus =  (threads * cores * sockets)
>>>>>>
>>>>>> static void smp_parse(MachineState *ms, QemuOpts *opts)
>>>>>> {
>>>>>>        [...]
>>>>>>
>>>>>>           if (sockets * cores * threads != ms->smp.max_cpus) {
>>>>>>               warn_report("Invalid CPU topology deprecated: "
>>>>>>                           "sockets (%u) * cores (%u) * threads (%u) "
>>>>>>                           "!= maxcpus (%u)",
>>>>>>                           sockets, cores, threads,
>>>>>>                           ms->smp.max_cpus);
>>>>>>           }
>>>>>>        [...]
>>>>>> }
>>>>>>
>>>>>> Although, above check does not exit(1) and just warns on detecting invalid
>>>>>> CPU topology. Not sure why?
>>>>> Hmm, not sure what code you have there. I see this in
>>>>> hw/core/machine.c:smp_parse
>>>>>
>>>>>           if (ms->smp.max_cpus < cpus) {
>>>>>               error_report("maxcpus must be equal to or greater than smp");
>>>>>               exit(1);
>>>>>           }
>>>>>
>>>>>           if (sockets * cores * threads != ms->smp.max_cpus) {
>>>>>               error_report("Invalid CPU topology: "
>>>>>                            "sockets (%u) * cores (%u) * threads (%u) "
>>>>>                            "!= maxcpus (%u)",
>>>>>                            sockets, cores, threads,
>>>>>                            ms->smp.max_cpus);
>>>>>               exit(1);
>>>>>           }
>>>>>
>>>>>> Well if you think there are subtleties to support above implementation and
>>>>>> we cannot do it now then sure it is your call. :)
>>> Hi Salil, Drew,
>>>>> The problem is that -smp 4,maxcpus=8 doesn't error out today, even though
>>>>> it doesn't do anything. OTOH, -smp 4,cores=2 doesn't error out either, but
>>>>> we're proposing that it should. Maybe we can start erroring out when
>>>>> cpus != maxcpus until hot plug is supported?
>>>> Agreed, both don't make any sense if hotplug is not supported and ideally should
>>>> fail with error. We should block any such topology configuration.
>>> In the ARM-specific function virt_smp_parse() (patch 9), there already
>>> have been some restrictions for the given -smp configuration.
>>> We now only allow:
>>> -smp N
>>> -smp maxcpus=M
>>> -smp N, maxcpus=M
>>>
>>> -smp N, sockets=X, cores=Y
>>> -smp N, sockets=X, cores=Y, threads=Z
>>>
>>> -smp maxcpus=M, sockets=X, cores=Y
>>> -smp maxcpus=M, sockets=X, cores=Y, threads=Z
>>>
>>> -smp N, maxcpus=M, sockets=X, cores=Y
>>> -smp N, maxcpus=M, sockets=X, cores=Y, threads=Z
>>>
>>> and disallow the other strange and rare formats that shouldn't be provided.
>>>
>>> It's reasonable to block the topology configuration which is not useful
>>> currently. I will add the requirement for "cpus==maxcpus" in this fuction
>>> if the possible conflict with existing command lines is not a big problem.
>> Hi Yanan,
>> Makes sense. I did see your other patch-set in which cluster support has been
>> added. Are we deferring that too?
> The merge of that needs to be deferred, but for a different reason. It
> shouldn't impact hot plug, because if hot plug doesn't like clusters,
> then one could configure a topology which doesn't have clusters. But,
> it can't be merged to QEMU until the kernel has merged its support.
Agreed!

Thanks,
Yanan
> Thanks,
> drew
>
> .