On Tue, Jan 16, 2018 at 10:20:18AM +0100, Greg Kurz wrote: > On Tue, 16 Jan 2018 15:47:13 +1100 > David Gibson <da...@gibson.dropbear.id.au> wrote: > > > At present if we require a vsmt mode that's not equal to the kernel's > > default, and the kernel doesn't let us change it (e.g. because it's an old > > kernel without support) then we always fail. > > > > But in fact we can cope with the kernel having a different vsmt as long as > > a) it's >= the actual number of vthreads/vcore (so that guest threads > > that are supposed to be on the same core act like it) > > b) it's a submultiple of the requested vsmt mode (so that guest threads > > spaced by the vsmt value will act like they're on different cores) > > > > Allowing this case gives us a bit more freedom to adjust the vsmt behaviour > > without breaking existing cases. > > > > Signed-off-by: David Gibson <da...@gibson.dropbear.id.au> > > --- > > I could check the following on a POWER9 host: > > $ ./ppc64-softmmu/qemu-system-ppc64 -accel kvm -smp threads=1 > qemu-system-ppc64: warning: Failed to set KVM's VSMT mode to 8 (errno -22) > > and the guest boots. > > $ ./ppc64-softmmu/qemu-system-ppc64 -accel kvm -smp threads=2 > qemu-system-ppc64: Failed to set KVM's VSMT mode to 8 (errno -22) > On PPC, a VM with 2 threads/core on a host with 1 threads/core requires the > use of VSMT mode 8. > This KVM seems to be too old to support VSMT. > > and QEMU exits.
I assume the above is with an old kernel that doesn't have the ability to set the SMT cap? > > Tested-by: Greg Kurz <gr...@kaod.org> > > Just one minor remark below but anyway: > > Reviewed-by: Greg Kurz <gr...@kaod.org> > > > hw/ppc/spapr.c | 26 +++++++++++++++++++------- > > 1 file changed, 19 insertions(+), 7 deletions(-) > > > > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c > > index e35214bfc3..6d3613d934 100644 > > --- a/hw/ppc/spapr.c > > +++ b/hw/ppc/spapr.c > > @@ -2314,17 +2314,29 @@ static void spapr_set_vsmt_mode(sPAPRMachineState > > *spapr, Error **errp) > > if (kvm_enabled() && (spapr->vsmt != kvm_smt)) { > > ret = kvmppc_set_smt_threads(spapr->vsmt); > > if (ret) { > > + /* Looks like KVM isn't able to change VSMT mode */ > > error_setg(&local_err, > > "Failed to set KVM's VSMT mode to %d (errno %d)", > > spapr->vsmt, ret); > > - if (!vsmt_user) { > > - error_append_hint(&local_err, "On PPC, a VM with %d > > threads/" > > - "core on a host with %d threads/core requires > > " > > - " the use of VSMT mode %d.\n", > > - smp_threads, kvm_smt, spapr->vsmt); > > + /* We can live with that if the default one is big enough > > + * for the number of threads, and a submultiple of the one > > + * we want. In this case we'll waste some vcpu ids, but > > + * behaviour will be correct */ > > + if ((kvm_smt >= smp_threads) && (spapr->vsmt % kvm_smt) == 0) { > > Inconsistent use of parens in the left and right operands of && > > > + warn_report_err(local_err); > > + local_err = NULL; > > + goto out; > > + } else { > > + if (!vsmt_user) { > > + error_append_hint(&local_err, > > + "On PPC, a VM with %d threads/core" > > + " on a host with %d threads/core" > > + " requires the use of VSMT mode > > %d.\n", > > + smp_threads, kvm_smt, spapr->vsmt); > > + } > > + kvmppc_hint_smt_possible(&local_err); > > + goto out; > > } > > - kvmppc_hint_smt_possible(&local_err); > > - goto out; > > } > > } > > /* else TCG: nothing to do currently */ > -- 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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