Re: [Qemu-devel] [PATCHv2 1/2] spapr: Allow some cases where we can't set VSMT mode in the kernel

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Jan 16, 2018 at 11:34:52AM +0100, Laurent Vivier wrote:
> On 16/01/2018 05:47, David Gibson 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 <david@gibson.dropbear.id.au>
> > ---
> >  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) {
> 
> I agree with Greg: inconsistent use of parenthesis, should be
> 
>     if (kvm_smt >= smp_threads && (spapr->vsmt % kvm_smt) == 0) {

Corrected in my tree.

> Anyway:
> 
> Reviewed-by: Laurent Vivier <lvivier@redhat.com>
> 
> Thanks,
> Laurent
> 

-- 
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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