spapr: Adjust default VSMT value for better migration compatibility

fa98fbfc "PC: KVM: Support machine option to set VSMT mode" introduced the
"vsmt" parameter for the pseries machine type, which controls the spacing
of the vcpu ids of thread 0 for each virtual core.  This was done to bring
some consistency and stability to how that was done, while still allowing
backwards compatibility for migration and otherwise.

The default value we used for vsmt was set to the max of the host's
advertised default number of threads and the number of vthreads per vcore
in the guest.  This was done to continue running without extra parameters
on older KVM versions which don't allow the VSMT value to be changed.

Unfortunately, even that smaller than before leakage of host configuration
into guest visible configuration still breaks things.  Specifically a guest
with 4 (or less) vthread/vcore will get a different vsmt value when
running on a POWER8 (vsmt==8) and POWER9 (vsmt==4) host.  That means the
vcpu ids don't line up so you can't migrate between them, though you should
be able to.

Long term we really want to make vsmt == smp_threads for sufficiently
new machine types.  However, that means that qemu will then require a
sufficiently recent KVM (one which supports changing VSMT) - that's still
not widely enough deployed to be really comfortable to do.

In the meantime we need some default that will work as often as
possible.  This patch changes that default to 8 in all circumstances.
This does change guest visible behaviour (including for existing
machine versions) for many cases - just not the most common/important
case.

Following is case by case justification for why this is still the least
worst option.  Note that any of the old behaviours can still be duplicated
after this patch, it's just that it requires manual intervention by
setting the vsmt property on the command line.

KVM HV on POWER8 host:
   This is the overwhelmingly common case in production setups, and is
   unchanged by design.  POWER8 hosts will advertise a default VSMT mode
   of 8, and > 8 vthreads/vcore isn't permitted

KVM HV on POWER7 host:
   Will break, but POWER7s allowing KVM were never released to the public.

KVM HV on POWER9 host:
   Not yet released to the public, breaking this now will reduce other
   breakage later.

KVM HV on PowerPC 970:
   Will theoretically break it, but it was barely supported to begin with
   and already required various user visible hacks to work.  Also so old
   that I just don't care.

TCG:
   This is the nastiest one; it means migration of TCG guests (without
   manual vsmt setting) will break.  Since TCG is rarely used in production
   I think this is worth it for the other benefits.  It does also remove
   one more barrier to TCG<->KVM migration which could be interesting for
   debugging applications.

KVM PR:
   As with TCG, this will break migration of existing configurations,
   without adding extra manual vsmt options.  As with TCG, it is rare in
   production so I think the benefits outweigh breakages.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
Reviewed-by: Greg Kurz <groug@kaod.org>

hw/ppc/spapr.c

@@ -2305,9 +2305,14 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
         }
         /* In this case, spapr->vsmt has been set by the command line */
     } else {
-        /* Choose a VSMT mode that may be higher than necessary but is
-         * likely to be compatible with hosts that don't have VSMT. */
-        spapr->vsmt = MAX(kvm_smt, smp_threads);
+        /*
+         * Default VSMT value is tricky, because we need it to be as
+         * consistent as possible (for migration), but this requires
+         * changing it for at least some existing cases.  We pick 8 as
+         * the value that we'd get with KVM on POWER8, the
+         * overwhelmingly common case in production systems.
+         */
+        spapr->vsmt = 8;
     }
 
     /* KVM: If necessary, set the SMT mode: */