On Fri, Feb 09, 2018 at 03:06:49PM +0100, Greg Kurz wrote:
> On Fri, 9 Feb 2018 09:18:58 +0100
> Laurent Vivier <lviv...@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.
>
> 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 <gr...@kaod.org>
>
> > Fixes: 8904e5a750
> > ("spapr: Adjust default VSMT value for better migration compatibility")
> >
> > Signed-off-by: Laurent Vivier <lviv...@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: */
>
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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