Re: [PATCH v20 08/21] qapi/s390x/cpu topology: set-cpu-topology qmp command

[Nina Schoetterl-Glausch <nsg@linux.ibm.com>]

On Tue, 2023-04-25 at 18:14 +0200, Pierre Morel wrote:
> The modification of the CPU attributes are done through a monitor
> command.
> 
> It allows to move the core inside the topology tree to optimize
> the cache usage in the case the host's hypervisor previously
> moved the CPU.
> 
> The same command allows to modify the CPU attributes modifiers
> like polarization entitlement and the dedicated attribute to notify
> the guest if the host admin modified scheduling or dedication of a vCPU.
> 
> With this knowledge the guest has the possibility to optimize the
> usage of the vCPUs.
> 
> The command has a feature unstable for the moment.
> 
> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>

Logic is sound, minor stuff below.

> ---
>  qapi/machine-target.json |  37 +++++++++++
>  hw/s390x/cpu-topology.c  | 136 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 173 insertions(+)
> 
> diff --git a/qapi/machine-target.json b/qapi/machine-target.json
> index 42a6a40333..3b7a0b77f4 100644
> --- a/qapi/machine-target.json
> +++ b/qapi/machine-target.json
> @@ -4,6 +4,8 @@
>  # This work is licensed under the terms of the GNU GPL, version 2 or later.
>  # See the COPYING file in the top-level directory.
>  
> +{ 'include': 'machine-common.json' }
> +
>  ##
>  # @CpuModelInfo:
>  #
> @@ -354,3 +356,38 @@
>  { 'enum': 'CpuS390Polarization',
>    'prefix': 'S390_CPU_POLARIZATION',
>    'data': [ 'horizontal', 'vertical' ] }
> +
> +##
> +# @set-cpu-topology:
> +#
> +# @core-id: the vCPU ID to be moved
> +# @socket-id: optional destination socket where to move the vCPU
> +# @book-id: optional destination book where to move the vCPU
> +# @drawer-id: optional destination drawer where to move the vCPU
> +# @entitlement: optional entitlement
> +# @dedicated: optional, if the vCPU is dedicated to a real CPU
> +#
> +# Features:
> +# @unstable: This command may still be modified.
> +#
> +# Modifies the topology by moving the CPU inside the topology
> +# tree or by changing a modifier attribute of a CPU.
> +# Default value for optional parameter is the current value
> +# used by the CPU.
> +#
> +# Returns: Nothing on success, the reason on failure.
> +#
> +# Since: 8.1
> +##
> +{ 'command': 'set-cpu-topology',
> +  'data': {
> +      'core-id': 'uint16',
> +      '*socket-id': 'uint16',
> +      '*book-id': 'uint16',
> +      '*drawer-id': 'uint16',
> +      '*entitlement': 'CpuS390Entitlement',
> +      '*dedicated': 'bool'
> +  },
> +  'features': [ 'unstable' ],
> +  'if': { 'all': [ 'TARGET_S390X' , 'CONFIG_KVM' ] }
> +}
> diff --git a/hw/s390x/cpu-topology.c b/hw/s390x/cpu-topology.c
> index d9cd3dc3ce..e5fb976594 100644
> --- a/hw/s390x/cpu-topology.c
> +++ b/hw/s390x/cpu-topology.c
> @@ -16,6 +16,7 @@
>  #include "target/s390x/cpu.h"
>  #include "hw/s390x/s390-virtio-ccw.h"
>  #include "hw/s390x/cpu-topology.h"
> +#include "qapi/qapi-commands-machine-target.h"
>  
>  /*
>   * s390_topology is used to keep the topology information.
> @@ -261,6 +262,27 @@ static bool s390_topology_check(uint16_t socket_id, uint16_t book_id,
>      return true;
>  }
>  
> +/**
> + * s390_topology_need_report
> + * @cpu: Current cpu
> + * @drawer_id: future drawer ID
> + * @book_id: future book ID
> + * @socket_id: future socket ID

Entitlement and dedicated are missing here.

> + *
> + * A modified topology change report is needed if the topology
> + * tree or the topology attributes change.
> + */
> +static int s390_topology_need_report(S390CPU *cpu, int drawer_id,

I'd prefer a bool return type.

> +                                   int book_id, int socket_id,
> +                                   uint16_t entitlement, bool dedicated)
> +{
> +    return cpu->env.drawer_id != drawer_id ||
> +           cpu->env.book_id != book_id ||
> +           cpu->env.socket_id != socket_id ||
> +           cpu->env.entitlement != entitlement ||
> +           cpu->env.dedicated != dedicated;
> +}
> +
>  /**
>   * s390_update_cpu_props:
>   * @ms: the machine state
> @@ -330,3 +352,117 @@ void s390_topology_setup_cpu(MachineState *ms, S390CPU *cpu, Error **errp)
>      /* topology tree is reflected in props */
>      s390_update_cpu_props(ms, cpu);
>  }
> +
> +static void s390_change_topology(uint16_t core_id,
> +                                 bool has_socket_id, uint16_t socket_id,
> +                                 bool has_book_id, uint16_t book_id,
> +                                 bool has_drawer_id, uint16_t drawer_id,
> +                                 bool has_entitlement, uint16_t entitlement,

I would keep the enum type for entitlement.

> +                                 bool has_dedicated, bool dedicated,
> +                                 Error **errp)
> +{
> +    MachineState *ms = current_machine;
> +    int old_socket_entry;
> +    int new_socket_entry;
> +    int report_needed;
> +    S390CPU *cpu;
> +    ERRP_GUARD();
> +
> +    if (core_id >= ms->smp.max_cpus) {
> +        error_setg(errp, "Core-id %d out of range!", core_id);
> +        return;
> +    }
> +
> +    cpu = (S390CPU *)ms->possible_cpus->cpus[core_id].cpu;

You can replace this with

       cpu = s390_cpu_addr2state(core_id);

and get rid of the if above that checks for out of range.

> +    if (!cpu) {
> +        error_setg(errp, "Core-id %d does not exist!", core_id);
> +        return;
> +    }
> +
> +    /* Get attributes not provided from cpu and verify the new topology */
> +    if (!has_socket_id) {
> +        socket_id = cpu->env.socket_id;
> +    }
> +    if (!has_book_id) {
> +        book_id = cpu->env.book_id;
> +    }
> +    if (!has_drawer_id) {
> +        drawer_id = cpu->env.drawer_id;
> +    }
> +    if (!has_dedicated) {
> +        dedicated = cpu->env.dedicated;
> +    }
> +
> +    /*
> +     * When the user specifies the entitlement as 'auto' on the command line,
> +     * qemu will set the entitlement as:
> +     * Medium when the CPU is not dedicated.
> +     * High when dedicated is true.
> +     */
> +    if (!has_entitlement || (entitlement == S390_CPU_ENTITLEMENT_AUTO)) {
> +        if (dedicated) {
> +            entitlement = S390_CPU_ENTITLEMENT_HIGH;
> +        } else {
> +            entitlement = S390_CPU_ENTITLEMENT_MEDIUM;
> +        }
> +    }
> +
> +    if (!s390_topology_check(socket_id, book_id, drawer_id,
> +                             entitlement, dedicated, errp))
> +        return;
> +
> +    /* Check for space on new socket */
> +    old_socket_entry = s390_socket_nb(cpu);
> +    new_socket_entry = __s390_socket_nb(drawer_id, book_id, socket_id);
> +
> +    if (new_socket_entry != old_socket_entry) {
> +        if (s390_topology.cores_per_socket[new_socket_entry] >=
> +            s390_topology.smp->cores) {
> +            error_setg(errp, "No more space on this socket");
> +            return;
> +        }
> +        /* Update the count of cores in sockets */
> +        s390_topology.cores_per_socket[new_socket_entry] += 1;
> +        s390_topology.cores_per_socket[old_socket_entry] -= 1;
> +    }
> +
> +    /* Check if we will need to report the modified topology */
> +    report_needed = s390_topology_need_report(cpu, drawer_id, book_id,
> +                                              socket_id, entitlement,
> +                                              dedicated);
> +
> +    /* All checks done, report new topology into the vCPU */
> +    cpu->env.drawer_id = drawer_id;
> +    cpu->env.book_id = book_id;
> +    cpu->env.socket_id = socket_id;
> +    cpu->env.dedicated = dedicated;
> +    cpu->env.entitlement = entitlement;
> +
> +    /* topology tree is reflected in props */
> +    s390_update_cpu_props(ms, cpu);
> +
> +    /* Advertise the topology change */
> +    if (report_needed) {
> +        s390_cpu_topology_set_changed(true);
> +    }
> +}
> +
> +void qmp_set_cpu_topology(uint16_t core,
> +                         bool has_socket, uint16_t socket,
> +                         bool has_book, uint16_t book,
> +                         bool has_drawer, uint16_t drawer,
> +                         bool has_entitlement, CpuS390Entitlement entitlement,
> +                         bool has_dedicated, bool dedicated,
> +                         Error **errp)
> +{
> +    ERRP_GUARD();
> +
> +    if (!s390_has_topology()) {
> +        error_setg(errp, "This machine doesn't support topology");
> +        return;
> +    }
> +
> +    s390_change_topology(core, has_socket, socket, has_book, book,
> +                         has_drawer, drawer, has_entitlement, entitlement,
> +                         has_dedicated, dedicated, errp);
> +}