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

[Igor Mammedov]

On Tue, 4 Sep 2018 23:12:55 -0300
Eduardo Habkost <address@hidden> 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 <address@hidden>
> > ---
> > v5:
> >   - extend deprecation doc, adding that maxcpus should be multiple of
> >     present on CLI [sockets/cores/threads] options
> >     (Eduardo Habkost <address@hidden>)
> > v4:
> >   - missed dot comment, fix it with s/./,/ (Andrew Jones <address@hidden>)
> > v3:
> >   - more spelling fixes (Andrew Jones <address@hidden>)
> >   - place deprecation check after (sockets * cores * threads > max_cpus) 
> > check
> >     (Eduardo Habkost <address@hidden>)
> > v2:
> >   - spelling&&co fixes (Andrew Jones <address@hidden>)
> > ---
> >  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.
> >  
> > address@hidden -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:
> 
>   @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.
> > +  sockets * cores * threads == maxcpus
> 
> Minor: same as above.  I suggest:
> 
>   @address@hidden * @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.

> 
> These are just suggestions in case you are going to respin the series, not
> blockers.
>