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

[Salil Mehta]

> 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?

Thanks