Re: [PATCH for-6.2 v2 01/11] machine: Disallow specifying topology parameters as zero

[wangyanan (Y)]

On 2021/7/20 0:46, Daniel P. Berrangé wrote:
> On Mon, Jul 19, 2021 at 11:20:33AM +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.
> >
> > Suggested-by: Andrew Jones <drjones@redhat.com>
> > Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
> > ---
> >   hw/core/machine.c | 31 +++++++++++++++++++++++--------
> >   hw/i386/pc.c      | 29 +++++++++++++++++++++--------
> >   qapi/machine.json |  4 ++--
> >   3 files changed, 46 insertions(+), 18 deletions(-)
> >
> > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > index 775add0795..d73daa10f4 100644
> > --- a/hw/core/machine.c
> > +++ b/hw/core/machine.c
> > @@ -745,11 +745,25 @@ static void smp_parse(MachineState *ms, SMPConfiguration 
> > *config, Error **errp)
> >   {
> >       unsigned cpus    = config->has_cpus ? config->cpus : 0;
> >       unsigned sockets = config->has_sockets ? config->sockets : 0;
> > +    unsigned dies    = config->has_dies ? config->dies : 1;
> It looks odd to set dies=1 by default at initially, when everything
> else is set to 0.  I realize you're just copying existing pc_smp_parse
> code in this respect, but I feel like could benefit from a separate
> initialization with a comment to explain why we're hardcoding it
> to 1....
>
> >       unsigned cores   = config->has_cores ? config->cores : 0;
> >       unsigned threads = config->has_threads ? config->threads : 0;
> > +    unsigned maxcpus = config->has_maxcpus ? config->maxcpus : 0;
> > +
> > +    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");
> > +        return;
> > +    }
> >   
> > -    if (config->has_dies && config->dies != 0 && config->dies != 1) {
> > +    if (dies > 1) {
> >           error_setg(errp, "dies not supported by this machine's CPU topology");
> > +        return;
> >       }
> .... eg how about here adding
>
>       /* Never try to assign multiple dies when defaulting omitted topology */
>       if (dies == 0) {
>           dies = 1;
>       }
Yeah, I agree to default dies to 0 like the other parameters at initially
and then explicitly assign it to 1 if omitted here. But I think this 
explicit
assignment should be in pc_smp_parse, because dies is never used in
the calculation in smp_parse yet except the front sanity-check.

So I think what should be updated for this patch is:
1) default dies to 0 at initially both in smp_parse and pc_smp_parse
2) then explicitly assign dies to 1 if it's omitted in pc_smp_parse
> > diff --git a/qapi/machine.json b/qapi/machine.json
> > index c3210ee1fb..c11b2e6f73 100644
> > --- a/qapi/machine.json
> > +++ b/qapi/machine.json
> > @@ -1288,8 +1288,8 @@
> >   ##
> >   # @SMPConfiguration:
> >   #
> > -# Schema for CPU topology configuration.  "0" or a missing value lets
> > -# QEMU figure out a suitable value based on the ones that are provided.
> > +# Schema for CPU topology configuration. A missing value lets QEMU
> > +# figure out a suitable value based on the ones that are provided.
> Hmm, so we had actually documented that '0' had the same semantics
> as omitting a parameter. This was done in:
>
>    commit 1e63fe685804dfadddd643bf3860b1a59702d4bf
>    Author: Paolo Bonzini <pbonzini@redhat.com>
>    Date:   Thu Jun 17 17:53:06 2021 +0200
>
>      machine: pass QAPI struct to mc->smp_parse
>
> which hasn't been released yet.
>
> This was possible, but never documented, with the traditiaonl -smp
> impl before it was qapi-ified. I think that historical behaviour
> was simply a side effect of the QemuOpts impl rather than an
> intentional design, hence not documented.
Agreed.
> At the very least I think need to get rid of this bit of docs about
> "0" before this release, otherwise we'll have stronger need to
> consider a real deprecation process.
If the doc needs to be fixed right now, then I think we'd better resend this
single patch separately for 6.1, including the doc fix and also the related
sanity-check in the parsers. Right?

Thanks,
Yanan
.