Re: [Qemu-devel] [PATCH v5 1/2] vl.c deprecate incorrect CPUs topology

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 5 Sep 2018 10:45:12 -0300
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Wed, Sep 05, 2018 at 11:25:11AM +0200, Igor Mammedov wrote:
> > On Tue, 4 Sep 2018 23:12:55 -0300
> > Eduardo Habkost <ehabkost@redhat.com> wrote:
> >   
> > > On Tue, Sep 04, 2018 at 03:22:35PM +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>
> > > > ---
> > > > v5:
> > > >   - extend deprecation doc, adding that maxcpus should be multiple of
> > > >     present on CLI [sockets/cores/threads] options
> > > >     (Eduardo Habkost <ehabkost@redhat.com>)
> > > > v4:
> > > >   - missed dot comment, fix it with s/./,/ (Andrew Jones <drjones@redhat.com>)
> > > > v3:
> > > >   - more spelling fixes (Andrew Jones <drjones@redhat.com>)
> > > >   - place deprecation check after (sockets * cores * threads > max_cpus) check
> > > >     (Eduardo Habkost <ehabkost@redhat.com>)
> > > > v2:
> > > >   - spelling&&co fixes (Andrew Jones <drjones@redhat.com>)
> > > > ---
> > > >  qemu-deprecated.texi | 19 +++++++++++++++++++
> > > >  vl.c                 |  7 +++++++
> > > >  2 files changed, 26 insertions(+)
> > > > 
> > > > diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
> > > > index 87212b6..827c3ce 100644
> > > > --- a/qemu-deprecated.texi
> > > > +++ b/qemu-deprecated.texi
> > > > @@ -159,6 +159,25 @@ 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)  
> > > 
> > > Minor: I suggest using @var markup, or maybe just use
> > > "-smp (invalid topologies) (since 3.1)" as subsection title for simplicity?
> > >   
> > > > +
> > > > +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  
> > > 
> > > Minor: this line formatting is lost on the HTML output.  I
> > > suggest using @var and/or @math.
> > > 
> > > Minor: I suggest not using C syntax.
> > > 
> > > i.e. use something like:
> > > 
> > >   @math{@var{n} <= @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.
> > > > +  sockets * cores * threads == maxcpus  
> > > 
> > > Minor: same as above.  I suggest:
> > > 
> > >   @math{@var{sockets} * @var{cores} * @var{threads} = @var{maxcpus}}.
> > > 
> > >   
> > > > +Note: it's assumed that maxcpus value must be multiple of the topology
> > > > +options present on command line to avoid creating an invalid topology.
> > > > +If maxcpus isn't be multiple of present topology options then the condition
> > > > +(sockets * cores * threads == maxcpus) can't be satisfied and it will
> > > > +trigger deprecation warning which later will be converted to a error.
> > > > +If you get deprecation warning it's recommended to explicitly specify
> > > > +a correct topology to make warning go away and ensure that it will
> > > > +continue working in the future.  
> > > 
> > > I don't understand the purpose of the "Note:" section.  It seems to duplicate
> > > what was already said in the lines above it.  Can we just remove it?  
> > Well, didn't I say that no additional explanation needed in v4?
> > Note section was added per your suggestion to explicitly say that maxcpus
> > should be multiple of options present on CLI.  
> 
> You are right: I did ask for additional clarification on the
> documentation, because it was not clear that the warning can
> still appear even if some of the sockets/cores/threads options
> were omitted.  The note didn't make that clearer to me, though.
I can respin if necessary (we are not in hurry so no need to merge
something that would be removed immediately).
Could you suggest a text for clarification for me to include on respin?

> 
> In either case, this can be done on a follow-up patch if
> necessary.
> 
> Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
>