Re: [PATCH for-6.1 v2] machine: Disallow specifying topology parameters as zero

[wangyanan (Y)]

On 2021/7/23 16:02, Markus Armbruster wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
>
> > On Thu, Jul 22, 2021 at 11:43:26PM +0800, Yanan Wang 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.
> > >
> > > Suggested-by: Andrew Jones <drjones@redhat.com>
> > > Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
> > > ---
> > >   hw/core/machine.c | 14 ++++++++++++++
> > >   qapi/machine.json |  6 +++---
> > >   qemu-options.hx   | 12 +++++++-----
> > >   3 files changed, 24 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > > index 775add0795..db129d937b 100644
> > > --- a/hw/core/machine.c
> > > +++ b/hw/core/machine.c
> > > @@ -829,6 +829,20 @@ 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
> > > +     * 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, "parameters must be equal to or greater than one if 
> > > provided");
> > I'd suggest a slight tweak since when seen it lacks context:
> >
> > $ ./qemu-system-x86_64 -smp 4,cores=0,sockets=2
> > qemu-system-x86_64: parameters must be equal to or greater than one if provided
> >
> >
> >      error_setg(errp, "CPU topology parameters must be equal to or greater than one 
> > if provided");
> Let's scratch "if provided".
>
> I'd replace "must be equal to or greater than one" by "must be
> positive", or maybe "must be greater than zero".
How about we use "must be greater than zero" ?
After a grep search of these two sentences in QEMU, they both show up
in several places. "must be positive" always reports an invalid value that
is "< 0". While the check in this patch actually reject an invalid zero 
value.
> > > 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
> "positive" again.
Thanks,
Yanan
> > > +    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
> >
> > If you make the text changes, then feel free to add this when posting v2:
> >
> >   Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
> >   Tested-by: Daniel P. Berrangé <berrange@redhat.com>
> >   
> >
> >
> > Regards,
> > Daniel
> .