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

[Pierre Morel <pmorel@linux.ibm.com>]

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 of
> 
>       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