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().
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: */
