On Mon, Feb 12, 2018 at 12:11:27PM +0100, Greg Kurz wrote:
> On Sat, 10 Feb 2018 20:23:07 +1100
> David Gibson <da...@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 <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.
> >
>
> 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,
That seems like a good idea.
> - this function should ensure all pre-requisites to compute the vcpu_id are
> met (including trying to set VSMT) or return an error
I'm much more dubious about this. Checks that can be performed
passively I'm ok with, but I think something that looks like a simple
calculation helper shouldn't be having side-effects like poking at KVM
state.
--
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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