Re: [Qemu-devel] [PATCH v2] vl.c deprecate incorrect CPUs topology

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, Aug 27, 2018 at 03:53:26PM +0200, Igor Mammedov wrote:
> -smp [cpus],sockets/cores/threads[,maxcpus] should describe topology
> so that total number of logical CPUs [sockets * cores * threads]
> would be equal to [maxcpus], however historically we didn't have
> such check in QEMU and it is possible to start VM with an invalid
> topology.
> Deprecate invalid options combination so we can make sure that
> the topology VM started with is always correct in the future.
> Users with an invalid sockets/cores/threads/maxcpus values should
> fix their CLI to make sure that
>    [sockets * cores * threads] == [maxcpus]
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
> v2:
>   - spelling&&co fixes (Andrew Jones <drjones@redhat.com>)
> ---
>  qemu-deprecated.texi | 11 +++++++++++
>  vl.c                 |  6 ++++++
>  2 files changed, 17 insertions(+)
> 
> diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
> index 87212b6..b649b2e 100644
> --- a/qemu-deprecated.texi
> +++ b/qemu-deprecated.texi
> @@ -159,6 +159,17 @@ The 'file' driver for drives is no longer appropriate for character or host
>  devices and will only accept regular files (S_IFREG). The correct driver
>  for these file types is 'host_cdrom' or 'host_device' as appropriate.
>  
> +@subsection -smp X,[socket=a,core=b,thread=c],maxcpus=Y (since 3.1)
> +
> +CPU topology properties should describe whole machine topology including
> +possible CPUs. but historically it was possible to start QEMU with
> +an incorrect topology where
> +  sockets * cores * threads >= X && X < maxcpus
> +which could lead to incorrect topology enumeration by the guest.
> +Support for invalid topology will be removed, the user must ensure
> +topologies described with -smp includes all possible cpus, i.e.:
> +  sockets * cores * threads == maxcpus
> +
>  @section QEMU Machine Protocol (QMP) commands
>  
>  @subsection block-dirty-bitmap-add "autoload" parameter (since 2.12.0)
> diff --git a/vl.c b/vl.c
> index 5ba06ad..238f856 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -1238,6 +1238,12 @@ static void smp_parse(QemuOpts *opts)
>              exit(1);
>          }
>  
> +        if (sockets * cores * threads != max_cpus) {
> +            warn_report("Invalid CPU topology deprecated: "
> +                        "sockets (%u) * cores (%u) * threads (%u) "
> +                        "!= maxcpus (%u)",
> +                        sockets, cores, threads, max_cpus);
> +        }

We don't need to print the warning if
  sockets * cores * threads > max_cpus,
because it already triggers error_report()/exit() below.  I
suggest reordering the checks:

    if (sockets * cores * threads > max_cpus) {
        error_report(...);
        exit(1);
    }
    if (sockets * cores * threads != max_cpus) {
        warn_report(...)
    }


>          if (sockets * cores * threads > max_cpus) {
>              error_report("cpu topology: "
>                           "sockets (%u) * cores (%u) * threads (%u) > "
> -- 
> 2.7.4
> 

-- 
Eduardo