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

[Andrew Jones <drjones@redhat.com>]

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.

Thanks,
drew