Re: [PATCH v1 1/9] s390x: smp: s390x dedicated smp parsing

[Pierre Morel]

On 7/16/21 11:14 AM, Daniel P. Berrangé wrote:
> On Fri, Jul 16, 2021 at 10:54:08AM +0200, Cornelia Huck wrote:
> > On Wed, Jul 14 2021, Pierre Morel <pmorel@linux.ibm.com> wrote:
> >
> > > We need a s390x dedicated SMP parsing to handle s390x specificities.
> > >
> > > In this patch we only handle threads, cores and sockets for
> > > s390x:
> > > - do not support threads, we always have 1 single thread per core
> > > - the sockets are filled one after the other with the cores
> > >
> > > Both these handlings are different from the standard smp_parse
> > > functionement and reflect the CPU topology in the simple case
> > > where all CPU belong to the same book.
> > >
> > > Topology levels above sockets, i.e. books, drawers, are not
> > > considered at this stage and will be introduced in a later patch.
> > >
> > > Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> > > ---
> > >   hw/s390x/s390-virtio-ccw.c | 42 ++++++++++++++++++++++++++++++++++++++
> > >   1 file changed, 42 insertions(+)
> > >
> > > diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
> > > index e4b18aef49..899d3a4137 100644
> > > --- a/hw/s390x/s390-virtio-ccw.c
> > > +++ b/hw/s390x/s390-virtio-ccw.c
> > > @@ -582,6 +582,47 @@ static ram_addr_t s390_fixup_ram_size(ram_addr_t sz)
> > >       return newsz;
> > >   }
> > >   
> > > +/*
> > > + * In S390CCW machine we do not support threads for now,
> > > + * only sockets and cores.
> > > + */
> > > +static void s390_smp_parse(MachineState *ms, QemuOpts *opts)
> >
> > It seems you based this on an older version of the code? The current
> > signature of this function since 1e63fe685804 ("machine: pass QAPI
> > struct to mc->smp_parse") is
> >
> > void (*smp_parse)(MachineState *ms, SMPConfiguration *config, Error **errp);
> >
> > That affects your parsing, and also lets you get rid of the ugly exit(1)
> > statements.
> >
> > > +{
> > > +    unsigned cpus    = qemu_opt_get_number(opts, "cpus", 1);
> > > +    unsigned sockets = qemu_opt_get_number(opts, "sockets", 1);
> > > +    unsigned cores   = qemu_opt_get_number(opts, "cores", 1);
> > > +
> > > +    if (opts) {
> > > +        if (cpus == 0 || sockets == 0 || cores == 0) {
> >
> > This behaviour looks different from what we do for other targets: if you
> > specify the value as 0, a value is calculated from the other values;
> > here, you error out. It's probably not a good idea to differ.
>
> I increasingly worry that we're making a mistake by going down the
> route of having custom smp_parse implementations per target, as this
> is showing signs of inconsistent behaviour and error reportings. I
> think the differences / restrictions have granularity at a different
> level that is being tested in many cases too.
>
> Whether threads != 1 is valid will likely vary depending on what
> CPU model is chosen, rather than what architecture is chosen.
> The same is true for dies != 1. We're not really checking this
> closely even in x86 - for example I can request nonsense such
> as a 25 year old i486 CPU model with hyperthreading and multiple
> dies
>
>    qemu-system-x86_64 -cpu 486 -smp 16,cores=4,dies=2,threads=2
>
> In this patch, there is no error reporting if the user specifies
> dies != 1 or threads != 1 - it just silently ignores the request
> which is not good.

yes, I should change this

> Some machine types may have constraints on CPU sockets.
>
> This can of course all be handled by custom smp_parse impls, but
> this is ultimately going to lead to alot of duplicated and
> inconsistent logic I fear.
>
> I wonder if we would be better off having machine class callback
> that can report topology constraints for the current configuration,
> along lines ofsmp_constraints(MachineState *ms,
>
>       smp_constraints(MachineState *ms,
>                       int *max_sockets,
>                       int *max_dies,
>                       int *max_cores,
>                       int *max_threads)

I find the idee good, but what about making it really machine agnostic 
by removing names and using a generic

        smp_constraints(MachineState *ms,
                        int *nb_levels,
                        int *levels[]
                        );

Level can be replaced by another name like container.
The machine could also provide the level/container names according to 
its internal documentation.

Regards,
Pierre



> And then have only a single smp_parse impl that takes into account
> these constraints, to report errors / fill in missing fields / etc ?
>
> Regards,
> Daniel

--
Pierre Morel
IBM Lab Boeblingen