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>
8904e5a750