Re: [PATCH for-6.1 v3 1/1] machine: Disallow specifying topology parameters as zero

[Cornelia Huck]

On Fri, Jul 23 2021, Yanan Wang <wangyanan55@huawei.com> wrote:

> In the SMP configuration, we should either specify a topology
> parameter with a reasonable value (equal to or greater than 1)
> or just leave it omitted and QEMU will calculate its value.
> Configurations which explicitly specify the topology parameters
> as zero like "sockets=0" are meaningless, so disallow them.
>
> However, the commit 1e63fe685804d
> (machine: pass QAPI struct to mc->smp_parse) has documented that
> '0' has the same semantics as omitting a parameter in the qapi
> comment for SMPConfiguration. So this patch fixes the doc and
> also adds the corresponding sanity check in the smp parsers.
>
> Reviewed-by: Andrew Jones <drjones@redhat.com>
> Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
> Tested-by: Daniel P. Berrange <berrange@redhat.com>
> Suggested-by: Andrew Jones <drjones@redhat.com>
> Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
> ---
>  hw/core/machine.c | 15 +++++++++++++++
>  qapi/machine.json |  6 +++---
>  qemu-options.hx   | 12 +++++++-----
>  3 files changed, 25 insertions(+), 8 deletions(-)
>
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index 775add0795..d95e8b6903 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -829,6 +829,21 @@ static void machine_set_smp(Object *obj, Visitor *v, 
> const char *name,
>          return;
>      }
>  
> +    /*
> +     * The topology parameters must be specified equal to or great than one

s/great/greater/

> +     * or just omitted, explicit configuration like "cpus=0" is not allowed.
> +     */
> +    if ((config->has_cpus && config->cpus == 0) ||
> +        (config->has_sockets && config->sockets == 0) ||
> +        (config->has_dies && config->dies == 0) ||
> +        (config->has_cores && config->cores == 0) ||
> +        (config->has_threads && config->threads == 0) ||
> +        (config->has_maxcpus && config->maxcpus == 0)) {
> +        error_setg(errp, "CPU topology parameters must be equal to "
> +                   "or greater than one if provided");
> +        goto out_free;
> +    }
> +
>      mc->smp_parse(ms, config, errp);
>      if (errp) {
>          goto out_free;
> diff --git a/qapi/machine.json b/qapi/machine.json
> index c3210ee1fb..9272cb3cf8 100644
> --- a/qapi/machine.json
> +++ b/qapi/machine.json
> @@ -1288,8 +1288,8 @@
>  ##
>  # @SMPConfiguration:
>  #
> -# Schema for CPU topology configuration.  "0" or a missing value lets
> -# QEMU figure out a suitable value based on the ones that are provided.
> +# Schema for CPU topology configuration. A missing value lets QEMU
> +# figure out a suitable value based on the ones that are provided.
>  #
>  # @cpus: number of virtual CPUs in the virtual machine
>  #
> @@ -1297,7 +1297,7 @@
>  #
>  # @dies: number of dies per socket in the CPU topology
>  #
> -# @cores: number of cores per thread in the CPU topology
> +# @cores: number of cores per die in the CPU topology
>  #
>  # @threads: number of threads per core in the CPU topology
>  #
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 99ed5ec5f1..b0168f8c48 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -223,11 +223,13 @@ SRST
>      of computing the CPU maximum count.
>  
>      Either the initial CPU count, or at least one of the topology parameters
> -    must be specified. Values for any omitted parameters will be computed
> -    from those which are given. Historically preference was given to the
> -    coarsest topology parameters when computing missing values (ie sockets
> -    preferred over cores, which were preferred over threads), however, this
> -    behaviour is considered liable to change.
> +    must be specified. The specified parameters must be equal to or great

s/great/greater/

> +    than one, explicit configuration like "cpus=0" is not allowed. Values
> +    for any omitted parameters will be computed from those which are given.
> +    Historically preference was given to the coarsest topology parameters
> +    when computing missing values (ie sockets preferred over cores, which
> +    were preferred over threads), however, this behaviour is considered
> +    liable to change.
>  ERST
>  
>  DEF("numa", HAS_ARG, QEMU_OPTION_numa,

Reviewed-by: Cornelia Huck <cohuck@redhat.com>