Re: [Qemu-devel] [PATCH] hw: add .min_cpus and .default_cpus fields to machine_class

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Nov 07, 2017 at 11:48:13AM -0800, Alistair Francis wrote:
> On Tue, Nov 7, 2017 at 11:32 AM, Eduardo Habkost <ehabkost@redhat.com> wrote:
> > On Mon, Nov 06, 2017 at 04:54:54PM -0500, Emilio G. Cota wrote:
> >> On Mon, Nov 06, 2017 at 12:10:22 -0200, Eduardo Habkost wrote:
> >> > On Fri, Nov 03, 2017 at 06:24:07PM -0400, Emilio G. Cota wrote:
> >> > >  /**
> >> > >   * MachineClass:
> >> > >   * @max_cpus: maximum number of CPUs supported. Default: 1
> >> > > + * @force_max_cpus: if set, force the global max_cpus to match @max_cpus
> >> > >   * @min_cpus: minimum number of CPUs supported. Default: 1
> >> > >   * @default_cpus: number of CPUs instantiated if none are specified. Default: 1
> >> > >   * @get_hotplug_handler: this function is called during bus-less
> >> >
> >> > If we have a field containing the default value for smp_cpus
> >> > (default_cpus), what about a default_max_cpus field for the
> >> > default value of max_cpus?  This could make the rules a bit
> >> > easier to follow.
> >> >
> >> > (But I wonder if there's a way we can do this without introducing
> >> > another MachineClass field and making the initialization code
> >> > more complex.)
> >>
> >> I don't think the rules are too hard to follow, since they're trivial
> >> for most machines because the defaults are reasonable (i.e. they
> >> only set max_cpus, and everything else is set to 1). Some machines
> >> though are tricky in what can and cannot support, e.g. the Xilinx
> >> ones we've been dealing with in this thread.
> >>
> >> wrt .default_max_cpus instead of .force_max_cpus: it lets users
> >> shoot themselves in the foot, so I wouldn't go this route.
> >> For instance, what do we do if the user passes maxcpus < default_max_cpus?
> >> max_cpus would be set to maxcpus, yet they could still pass the
> >> -global override that instantiates two extra CPUs, and at that point
> >> we'd fail (in TCG) without providing a meaningful error.
> >>
> >> We could increase maxcpus to default_max_cpus, but at that point
> >> default_max_cpus becomes the same thing as force_max_cpus (!).
> >>
> >> An alternative I've come up with is to have a field that specifies how
> >> many CPUs could be created in addition to those specified in max_cpus
> >> (in the Xilinx case these would be the RPUs). Patch below.
> >>
> >> > The fact that MachineClass::max_cpus and the 'max_cpus' globals
> >> > have completely different meanings makes this confusing enough.
> >> > Maybe we should rename the 'max_cpus' global variable to
> >> > 'max_hotplug_cpus'.
> >>
> >> I have no trouble with their different meanings, although I admit
> >> I am now quite well acquainted with this code so my bias is strong :-)
> >>
> >> Thanks,
> >>
> >>               Emilio
> >>
> >> ---8<---
> >>
> >> Subject: [PATCH] hw: add .{additional_max,min,default}_cpus fields to
> >>  machine_class
> >>
> >> max_cpus needs to be an upper bound on the number of vCPUs
> >> initialized; otherwise TCG region initialization breaks.
> >>
> >> Some boards initialize a hard-coded number of vCPUs, which is not
> >> captured by the global max_cpus. Fix it by adding the .min_cpus
> >> field to machine_class.
> >>
> >> A subset of those boards can also instantiate additional vCPUs
> >> beyond what is specified in -smp, e.g. via -global overrides. To
> >> ensure that max_cpus remains an upper bound on the number of vCPUs
> >> that can be instantiated, we also add the .additional_max_cpus field
> >> to machine_class.
> >
> > Setting max_cpus > smp_cpus is supposed to be useful only for CPU
> > hotplug, and this doesn't seem to be the case here.  I would
> > prefer to make the CPU creation code not care about max_cpus at
> > all unless it is dealing with CPU hotplug, but this would require
> > auditing existing code that could be allocating data structures
> > based on max_cpus.
> >
> > In either case, it looks like the series sent by Alistair:
> >   Subject: [PATCH v1 0/2]  Xilinx ZCU102 fixes for 2.11
> > makes max_additional_cpus unnecessary, doesn't it?
> 
> Yes, it doesn't need the max_additional_cpus at all.
> 
> I would still like to see the default_cpus in, as I think that is
> really useful. Otherwise 2.10 has 4 A53s and 2.11 has 1 A53 by default
> which I think is confusing.

I agree with the addition of min_cpus and default_cpus.  But I
would like to avoid making xlnx-* the only exceptional cases
where max_additional_cpus is required, so I like the approach in
your series.

> 
> Thanks,
> Alistair
> 
> >
> >
> >>
> >> This commit also changes some user-facing behaviour: we now die if
> >> -smp is below this hard-coded vCPU minimum instead of silently
> >> ignoring the passed -smp value (sometimes announcing this by printing
> >> a warning). However, the introduction of .default_cpus lessens the
> >> likelihood that users will notice this: if -smp isn't set, we now
> >> assign the value in .default_cpus to both smp_cpus and max_cpus. IOW,
> >> if a user does not set -smp, they always get a correct number of vCPUs.
> >>
> >> This change fixes 3468b59 ("tcg: enable multiple TCG contexts in
> >> softmmu", 2017-10-24), which broke TCG initialization for some
> >> ARM boards.
> >>
> >> Fixes: 3468b59e18b179bc63c7ce934de912dfa9596122
> >> Reported-by: Thomas Huth <thuth@redhat.com>
> >> Suggested-by: Peter Maydell <peter.maydell@linaro.org>
> >> Signed-off-by: Emilio G. Cota <cota@braap.org>
> >> ---
> >>  hw/arm/exynos4_boards.c | 12 ++++--------
> >>  hw/arm/raspi.c          |  2 ++
> >>  hw/arm/xlnx-zcu102.c    |  9 ++++++++-
> >>  include/hw/boards.h     | 10 ++++++++++
> >>  vl.c                    | 38 +++++++++++++++++++++++++++++++++++---
> >>  5 files changed, 59 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/hw/arm/exynos4_boards.c b/hw/arm/exynos4_boards.c
> >> index f1441ec..750162c 100644
> >> --- a/hw/arm/exynos4_boards.c
> >> +++ b/hw/arm/exynos4_boards.c
> >> @@ -27,7 +27,6 @@
> >>  #include "qemu-common.h"
> >>  #include "cpu.h"
> >>  #include "sysemu/sysemu.h"
> >> -#include "sysemu/qtest.h"
> >>  #include "hw/sysbus.h"
> >>  #include "net/net.h"
> >>  #include "hw/arm/arm.h"
> >> @@ -129,13 +128,6 @@ exynos4_boards_init_common(MachineState *machine,
> >>                             Exynos4BoardType board_type)
> >>  {
> >>      Exynos4BoardState *s = g_new(Exynos4BoardState, 1);
> >> -    MachineClass *mc = MACHINE_GET_CLASS(machine);
> >> -
> >> -    if (smp_cpus != EXYNOS4210_NCPUS && !qtest_enabled()) {
> >> -        error_report("%s board supports only %d CPU cores, ignoring smp_cpus"
> >> -                     " value",
> >> -                     mc->name, EXYNOS4210_NCPUS);
> >> -    }
> >>
> >>      exynos4_board_binfo.ram_size = exynos4_board_ram_size[board_type];
> >>      exynos4_board_binfo.board_id = exynos4_board_id[board_type];
> >> @@ -189,6 +181,8 @@ static void nuri_class_init(ObjectClass *oc, void *data)
> >>      mc->desc = "Samsung NURI board (Exynos4210)";
> >>      mc->init = nuri_init;
> >>      mc->max_cpus = EXYNOS4210_NCPUS;
> >> +    mc->min_cpus = EXYNOS4210_NCPUS;
> >> +    mc->default_cpus = EXYNOS4210_NCPUS;
> >>      mc->ignore_memory_transaction_failures = true;
> >>  }
> >>
> >> @@ -205,6 +199,8 @@ static void smdkc210_class_init(ObjectClass *oc, void *data)
> >>      mc->desc = "Samsung SMDKC210 board (Exynos4210)";
> >>      mc->init = smdkc210_init;
> >>      mc->max_cpus = EXYNOS4210_NCPUS;
> >> +    mc->min_cpus = EXYNOS4210_NCPUS;
> >> +    mc->default_cpus = EXYNOS4210_NCPUS;
> >>      mc->ignore_memory_transaction_failures = true;
> >>  }
> >>
> >> diff --git a/hw/arm/raspi.c b/hw/arm/raspi.c
> >> index 5941c9f..cd5fa8c 100644
> >> --- a/hw/arm/raspi.c
> >> +++ b/hw/arm/raspi.c
> >> @@ -167,6 +167,8 @@ static void raspi2_machine_init(MachineClass *mc)
> >>      mc->no_floppy = 1;
> >>      mc->no_cdrom = 1;
> >>      mc->max_cpus = BCM2836_NCPUS;
> >> +    mc->min_cpus = BCM2836_NCPUS;
> >> +    mc->default_cpus = BCM2836_NCPUS;
> >>      mc->default_ram_size = 1024 * 1024 * 1024;
> >>      mc->ignore_memory_transaction_failures = true;
> >>  };
> >> diff --git a/hw/arm/xlnx-zcu102.c b/hw/arm/xlnx-zcu102.c
> >> index e2d15a1..0f1837e 100644
> >> --- a/hw/arm/xlnx-zcu102.c
> >> +++ b/hw/arm/xlnx-zcu102.c
> >> @@ -185,6 +185,10 @@ static void xlnx_ep108_machine_class_init(ObjectClass *oc, void *data)
> >>      mc->block_default_type = IF_IDE;
> >>      mc->units_per_default_bus = 1;
> >>      mc->ignore_memory_transaction_failures = true;
> >> +    mc->max_cpus = XLNX_ZYNQMP_NUM_APU_CPUS;
> >> +    mc->max_additional_cpus = XLNX_ZYNQMP_NUM_RPU_CPUS;
> >> +    mc->min_cpus = XLNX_ZYNQMP_NUM_APU_CPUS;
> >> +    mc->default_cpus = XLNX_ZYNQMP_NUM_APU_CPUS;
> >>  }
> >>
> >>  static const TypeInfo xlnx_ep108_machine_init_typeinfo = {
> >> @@ -240,7 +244,10 @@ static void xlnx_zcu102_machine_class_init(ObjectClass *oc, void *data)
> >>      mc->block_default_type = IF_IDE;
> >>      mc->units_per_default_bus = 1;
> >>      mc->ignore_memory_transaction_failures = true;
> >> -    mc->max_cpus = XLNX_ZYNQMP_NUM_APU_CPUS + XLNX_ZYNQMP_NUM_RPU_CPUS;
> >> +    mc->max_cpus = XLNX_ZYNQMP_NUM_APU_CPUS;
> >> +    mc->max_additional_cpus = XLNX_ZYNQMP_NUM_RPU_CPUS;
> >> +    mc->min_cpus = XLNX_ZYNQMP_NUM_APU_CPUS;
> >> +    mc->default_cpus = XLNX_ZYNQMP_NUM_APU_CPUS;
> >>  }
> >>
> >>  static const TypeInfo xlnx_zcu102_machine_init_typeinfo = {
> >> diff --git a/include/hw/boards.h b/include/hw/boards.h
> >> index 191a5b3..be582dd 100644
> >> --- a/include/hw/boards.h
> >> +++ b/include/hw/boards.h
> >> @@ -102,6 +102,13 @@ typedef struct {
> >>
> >>  /**
> >>   * MachineClass:
> >> + * @max_cpus: maximum number of CPUs supported. Default: 1
> >> + * @max_additional_cpus: # of CPUs that can be instantiated in addition to those
> >> + *    instantiated with -smp. For instance, if a machine has 4 main CPUs and 2
> >> + *    optional CPUs (e.g. instantiated via -global overrides), then @max_cpus
> >> + *    and @max_additional_cpus will be set to 4 and 2, respectively. Default: 0
> >> + * @min_cpus: minimum number of CPUs supported. Default: 1
> >> + * @default_cpus: number of CPUs instantiated if none are specified. Default: 1
> >>   * @get_hotplug_handler: this function is called during bus-less
> >>   *    device hotplug. If defined it returns pointer to an instance
> >>   *    of HotplugHandler object, which handles hotplug operation
> >> @@ -167,6 +174,9 @@ struct MachineClass {
> >>      BlockInterfaceType block_default_type;
> >>      int units_per_default_bus;
> >>      int max_cpus;
> >> +    int max_additional_cpus;
> >> +    int min_cpus;
> >> +    int default_cpus;
> >>      unsigned int no_serial:1,
> >>          no_parallel:1,
> >>          use_virtcon:1,
> >> diff --git a/vl.c b/vl.c
> >> index ec29909..e3ee36e 100644
> >> --- a/vl.c
> >> +++ b/vl.c
> >> @@ -160,8 +160,8 @@ Chardev *virtcon_hds[MAX_VIRTIO_CONSOLES];
> >>  Chardev *sclp_hds[MAX_SCLP_CONSOLES];
> >>  int win2k_install_hack = 0;
> >>  int singlestep = 0;
> >> -int smp_cpus = 1;
> >> -unsigned int max_cpus = 1;
> >> +int smp_cpus;
> >> +unsigned int max_cpus;
> >>  int smp_cores = 1;
> >>  int smp_threads = 1;
> >>  int acpi_enabled = 1;
> >> @@ -4330,12 +4330,44 @@ int main(int argc, char **argv, char **envp)
> >>      smp_parse(qemu_opts_find(qemu_find_opts("smp-opts"), NULL));
> >>
> >>      machine_class->max_cpus = machine_class->max_cpus ?: 1; /* Default to UP */
> >> +    machine_class->min_cpus = machine_class->min_cpus ?: 1;
> >> +    machine_class->default_cpus = machine_class->default_cpus ?: 1;
> >> +
> >> +    /* if -smp is not set, default to mc->default_cpus */
> >> +    if (!smp_cpus) {
> >> +        smp_cpus = machine_class->default_cpus;
> >> +        max_cpus = machine_class->default_cpus;
> >> +    }
> >> +
> >> +    /* sanity-check smp_cpus and max_cpus */
> >> +    if (smp_cpus < machine_class->min_cpus) {
> >> +        error_report("Invalid SMP CPUs %d. The min CPUs "
> >> +                     "supported by machine '%s' is %d", smp_cpus,
> >> +                     machine_class->name, machine_class->min_cpus);
> >> +        exit(1);
> >> +    }
> >>      if (max_cpus > machine_class->max_cpus) {
> >> -        error_report("Invalid SMP CPUs %d. The max CPUs "
> >> +        error_report("Invalid max SMP CPUs %d. The max CPUs "
> >>                       "supported by machine '%s' is %d", max_cpus,
> >>                       machine_class->name, machine_class->max_cpus);
> >>          exit(1);
> >>      }
> >> +    if (max_cpus < machine_class->min_cpus) {
> >> +        error_report("Invalid max SMP CPUs %d. The min CPUs "
> >> +                     "supported by machine '%s' is %d", max_cpus,
> >> +                     machine_class->name, machine_class->min_cpus);
> >> +        exit(1);
> >> +    }
> >> +
> >> +    /*
> >> +     * Some boards can instantiate additional CPUs, e.g. by overriding
> >> +     * device params via -global arguments. Update max_cpus so that
> >> +     * we are sure to have an upper bound on the maximum number of
> >> +     * vCPUs that might be instantiated.
> >> +     */
> >> +    if (machine_class->max_additional_cpus) {
> >> +        max_cpus += machine_class->max_additional_cpus;
> >> +    }
> >>
> >>      /*
> >>       * Get the default machine options from the machine if it is not already
> >> --
> >> 2.7.4
> >>
> >
> > --
> > Eduardo
> >

-- 
Eduardo