RE: [RFC PATCH v3 4/4] hw/arm/virt: Parse -smp cluster parameter in virt_smp_parse

[Salil Mehta]

> From: wangyanan (Y)
> 
> Hi Salil,
> 
> On 2021/5/17 23:17, Salil Mehta wrote:
> >> From: Qemu-devel
> >> [mailto:qemu-devel-bounces+salil.mehta=huawei.com@xxxxxxxxxx] On Behalf Of
> >> Yanan Wang
> >> Sent: Sunday, May 16, 2021 11:32 AM
> >> To: Peter Maydell <peter.maydell@xxxxxxxxxx>; Paolo Bonzini
> >> <pbonzini@xxxxxxxxxx>; Andrew Jones <drjones@xxxxxxxxxx>; Michael S . Tsirkin
> >> <mst@xxxxxxxxxx>; Igor Mammedov <imammedo@xxxxxxxxxx>; Shannon Zhao
> >> <shannon.zhaosl@xxxxxxxxx>; qemu-devel@xxxxxxxxxx; qemu-arm@xxxxxxxxxx
> >> Cc: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>; Philippe
> >> Mathieu-Daudé <philmd@xxxxxxxxxx>; wangyanan (Y) <wangyanan55@xxxxxxxxxx>;
> >> Zengtao (B) <prime.zeng@xxxxxxxxxxxxx>; Wanghaibin (D)
> >> <wanghaibin.wang@xxxxxxxxxx>; yuzenghui <yuzenghui@xxxxxxxxxx>; yangyicong
> >> <yangyicong@xxxxxxxxxx>; zhukeqian <zhukeqian1@xxxxxxxxxx>
> >> Subject: [RFC PATCH v3 4/4] hw/arm/virt: Parse -smp cluster parameter in
> >> virt_smp_parse
> >>
> >> There is a separate function virt_smp_parse() in hw/virt/arm.c used
> >> to parse cpu topology for the ARM machines. So add parsing of -smp
> >> cluster parameter in it, then total number of logical cpus will be
> >> calculated like: max_cpus = sockets * clusters * cores * threads.
> >>
> >> Note, we will assume multi-cluster in one socket is not supported
> >> and default the value of clusters to 1, if it's not explicitly
> >> specified in -smp cmdline.
> >>
> >> Signed-off-by: Yanan Wang <wangyanan55@xxxxxxxxxx>
> >> ---
> >>   hw/arm/virt.c | 32 ++++++++++++++++++--------------
> >>   1 file changed, 18 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> >> index 7de822e491..678d5ef36c 100644
> >> --- a/hw/arm/virt.c
> >> +++ b/hw/arm/virt.c
> >> @@ -2642,8 +2642,8 @@ static int virt_kvm_type(MachineState *ms, const char
> >> *type_str)
> >>    * with the -smp cmdlines when parsing them.
> >>    *
> >>    * We require that at least one of cpus or maxcpus must be provided.
> >> - * Threads will default to 1 if not provided. Sockets and cores must
> >> - * be either both provided or both not.
> >> + * Clusters and threads will default to 1 if they are not provided.
> >> + * Sockets and cores must be either both provided or both not.
> >>    *
> >>    * Note, if neither sockets nor cores are specified, we will calculate
> >>    * all the missing values just like smp_parse() does, but will disable
> >> @@ -2652,15 +2652,18 @@ static int virt_kvm_type(MachineState *ms, const char
> >> *type_str)
> >>   static void virt_smp_parse(MachineState *ms, QemuOpts *opts)
> >>   {
> >>       VirtMachineClass *vmc = VIRT_MACHINE_GET_CLASS(ms);
> >> +    VirtMachineState *vms = VIRT_MACHINE(ms);
> >>
> >>       if (opts) {
> >>           unsigned cpus = qemu_opt_get_number(opts, "cpus", 0);
> >>           unsigned maxcpus = qemu_opt_get_number(opts, "maxcpus", 0);
> >>           unsigned sockets = qemu_opt_get_number(opts, "sockets", 0);
> >> +        unsigned clusters = qemu_opt_get_number(opts, "clusters", 0);
> >>           unsigned cores = qemu_opt_get_number(opts, "cores", 0);
> >>           unsigned threads = qemu_opt_get_number(opts, "threads", 0);
> >>
> >> -        /* Default threads to 1 if not provided */
> >> +        /* Default clusters and threads to 1 if not provided */
> >> +        clusters = clusters > 0 ? clusters : 1;
> >>           threads = threads > 0 ? threads : 1;
> >>
> >>           if (cpus == 0 && maxcpus == 0) {
> >> @@ -2676,13 +2679,13 @@ static void virt_smp_parse(MachineState *ms, QemuOpts
> >> *opts)
> >>               cores = 1;
> >>               if (cpus == 0) {
> >>                   sockets = 1;
> >> -                cpus = sockets * cores * threads;
> >> +                cpus = sockets * clusters * cores * threads;
> >>               } else {
> >>                   maxcpus = maxcpus > 0 ? maxcpus : cpus;
> >> -                sockets = maxcpus / (cores * threads);
> >> +                sockets = maxcpus / (clusters * cores * threads);
> >>               }
> >>           } else if (sockets > 0 && cores > 0) {
> >> -            cpus = cpus > 0 ? cpus : sockets * cores * threads;
> >> +            cpus = cpus > 0 ? cpus : sockets * clusters * cores * threads;
> >>               maxcpus = maxcpus > 0 ? maxcpus : cpus;
> >>           } else {
> >>               error_report("sockets and cores must be both provided "
> >> @@ -2695,25 +2698,26 @@ static void virt_smp_parse(MachineState *ms, QemuOpts
> >> *opts)
> >>               exit(1);
> >>           }
> >>
> >> -        if (sockets * cores * threads < cpus) {
> >> +        if (sockets * clusters * cores * threads < cpus) {
> >>               error_report("cpu topology: "
> >> -                         "sockets (%u) * cores (%u) * threads (%u) < "
> >> -                         "smp_cpus (%u)",
> >> -                         sockets, cores, threads, cpus);
> >> +                         "sockets (%u) * clusters (%u) * cores (%u) * "
> >> +                         "threads (%u) < smp_cpus (%u)",
> >> +                         sockets, clusters, cores, threads, cpus);
> >>               exit(1);
> >>           }
> >>
> >> -        if (sockets * cores * threads != maxcpus) {
> >> +        if (sockets * clusters * cores * threads != maxcpus) {
> >>               error_report("cpu topology: "
> >> -                         "sockets (%u) * cores (%u) * threads (%u) "
> >> -                         "!= maxcpus (%u)",
> >> -                         sockets, cores, threads, maxcpus);
> >> +                         "sockets (%u) * clusters (%u) * cores (%u) * "
> >> +                         "threads (%u) != maxcpus (%u)",
> >> +                         sockets, clusters, cores, threads, maxcpus);
> >>               exit(1);
> >>           }
> >>
> >>           ms->smp.cpus = cpus;
> >>           ms->smp.max_cpus = maxcpus;
> >>           ms->smp.sockets = sockets;
> >> +        vms->smp_clusters = clusters;
> >
> > This variable naming *smp_clusters* looks out-of-sorts. I thought a similar
> > variable *smp_cpus* was destined to be removed for the reason given in below
> > link - a patch by Andrew Jones?
> >
> > Link: https://lists.gnu.org/archive/html/qemu-arm/2020-12/msg00418.html
> >
> > Am I missing anything here?
> The smp_clusters is added in VirtMachineState and nowhere else because
> it's currently only used for ARM. But I think maybe I should also move it to
> CpuTopology structure like [1] is doing to move dies to CpuTopology.

yes, that’s the idea. It is always good to have right place holders so that
the code comprehension/usage(in this case) becomes easy and obvious. 


> 
> Move clusters to CpuTopology won't affect other architectures that don't
> support it yet, and will also make it easy if they want to in the future.
> 
> [1] From Paolo:
> https://patchwork.kernel.org/project/qemu-devel/patch/20210513162901.131023
> 9-10-pbonzini@xxxxxxxxxx/

sure.


> 
> Thanks,
> Yanan
> > Salil.
> >
> >>           ms->smp.cores = cores;
> >>           ms->smp.threads = threads;
> >>       }