Re: [Qemu-devel] [libvirt] [PATCH v7 1/2] vl.c deprecate incorrect CPUs topology

[Eric Blake]

On 9/12/18 11:19 AM, 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 <address@hidden>
> Reviewed-by: Andrew Jones <address@hidden>
> Reviewed-by: Eduardo Habkost <address@hidden>
> ---

> +++ b/qemu-deprecated.texi
> @@ -139,6 +139,19 @@ 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.
>   
> address@hidden -smp (invalid topologies) (since 3.1)
> +
> +CPU topology properties should describe whole machine topology including
> +possible CPUs. In other words: @var{maxcpus} should be equal to
> address@hidden@var{sockets} * @var{cores} * @var{threads}}.

This sentence...

> +
> +However, historically it was possible to start QEMU with an incorrect topology
> +where @address@hidden <= @var{sockets} * @var{cores} * @var{threads} < 
> @var{maxcpus}},
> +which could lead to an incorrect topology enumeration by the guest.
> +Support for invalid topologies will be removed, the user must ensure
> +topologies described with -smp include all possible cpus, i.e.
> +  @address@hidden * @var{cores} * @var{threads} = @var{maxcpus}}.

...and this recommendation sound repetitive.  I'd be okay if you removed 
the "In other words" sentence above, leaving just the intro statement 
(what -cpu is supposed to do), then the second paragraph (what we also 
allowed in the past, and how you should fix it before we stop allowing 
that).

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org