Re: [Qemu-devel] [PATCH v3] spapr: set vsmt to MAX(8, smp_threads)

[Greg Kurz <groug@kaod.org>]

On Sat, 10 Feb 2018 20:23:07 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Fri, Feb 09, 2018 at 03:06:49PM +0100, Greg Kurz wrote:
> > On Fri,  9 Feb 2018 09:18:58 +0100
> > Laurent Vivier <lvivier@redhat.com> wrote:
> >   
> > > We ignore silently the value of smp_threads when we set
> > > the default VSMT value, and if smp_threads is greater than VSMT
> > > kernel is going into trouble later.
> > >   
> > 
> > Hi Laurent,
> > 
> > I've looked a bit more and I'm not sure what kernel troubles you're referring to,
> > but several places in QEMU where we use kvm_ppc_smt() later on do assume that
> > smp_threads > kvm_ppc_smt(). Basically, everywhere we compute a vCPU id:
> > 
> > In spapr_init_cpus() when creating DRC connectors:
> > 
> >         int core_id = i * smp_threads;
> > 
> >         if (mc->has_hotpluggable_cpus) {
> >             spapr_dr_connector_new(OBJECT(spapr), TYPE_SPAPR_DRC_CPU,
> >                                    (core_id / smp_threads) * smt);
> >         }
> > 
> > or in spapr_cpu_core_realize() when creating vCPUs:
> > 
> >         cpu->vcpu_id = (cc->core_id * spapr->vsmt / smp_threads) + i;
> > 
> > It is visible by adding some printfs in the current code base. This is what
> > happens when passing -smp cores=2,threads=16 without your patch:
> > 
> > DRC connector to vcpu_id 0
> > CPU vcpu_id 0
> > CPU vcpu_id 1
> > CPU vcpu_id 2
> > CPU vcpu_id 3
> > CPU vcpu_id 4
> > CPU vcpu_id 5
> > CPU vcpu_id 6
> > CPU vcpu_id 7
> > CPU vcpu_id 8
> > CPU vcpu_id 9
> > CPU vcpu_id 10
> > CPU vcpu_id 11
> > CPU vcpu_id 12
> > CPU vcpu_id 13
> > CPU vcpu_id 14
> > CPU vcpu_id 15
> > DRC connector to vcpu_id 8
> >                         ^^^
> >                      should be 16
> > CPU vcpu_id 8
> >            ^^^
> >    should start numbering at 16
> > CPU vcpu_id 9
> > CPU vcpu_id 10
> > CPU vcpu_id 11
> > CPU vcpu_id 12
> > CPU vcpu_id 13
> > CPU vcpu_id 14
> > CPU vcpu_id 15
> > CPU vcpu_id 16
> > CPU vcpu_id 17
> > CPU vcpu_id 18
> > CPU vcpu_id 19
> > CPU vcpu_id 20
> > CPU vcpu_id 21
> > CPU vcpu_id 22
> > CPU vcpu_id 23
> > qemu-system-ppc64: kvm_init_vcpu failed: File exists
> >                                               ^^^^
> >                                  CPU 8 already created by the first core
> > 
> > I'm not feeling comfortable with the rest of the code silently depending on
> > the fact that spapr_set_vsmt_mode() terminates QEMU if it cannot enforce
> > smp_threads <= kvm_ppc_smt().  
> 
> I'm not quite sure what you're suggesting as an alternative, though.
> 

I haven't suggested anything yet :)

But I was thinking of:
- having a single function to compute the vcpu_id, instead of open-coding
  the formula in several places like the current code does,
- this function should ensure all pre-requisites to compute the vcpu_id are
  met (including trying to set VSMT) or return an error

> > 
> > Anyway, with your patch, the same command line as above gives:
> > 
> > qemu-system-ppc64: Failed to set KVM's VSMT mode to 16 (errno -22)
> > On PPC, a VM with 16 threads/core on a host with 8 threads/core requires
> >  the use of VSMT mode 16.
> > This KVM seems to be too old to support VSMT.
> > 
> > This hammer is big enough to fix the vCPU ids miscalculations, so:
> > 
> > Reviewed-by: Greg Kurz <groug@kaod.org>
> >   
> > > Fixes: 8904e5a750
> > > ("spapr: Adjust default VSMT value for better migration compatibility")
> > > 
> > > Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> > > ---
> > > 
> > > Notes:
> > >     v3: use MAX(8, smp_threads) and let KVM to return an error
> > >         if nb_threads is too big
> > >         update subject to reflect the change
> > >     
> > >     v2: display a specific error message when the default VSMT is used
> > >         fix subject
> > > 
> > >  hw/ppc/spapr.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > > index 32a876be56..c8a1eefa17 100644
> > > --- a/hw/ppc/spapr.c
> > > +++ b/hw/ppc/spapr.c
> > > @@ -2310,7 +2310,7 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
> > >           * the value that we'd get with KVM on POWER8, the
> > >           * overwhelmingly common case in production systems.
> > >           */
> > > -        spapr->vsmt = 8;
> > > +        spapr->vsmt = MAX(8, smp_threads);
> > >      }
> > >  
> > >      /* KVM: If necessary, set the SMT mode: */  
> >   
>