On 15/01/2018 08:27, David Gibson wrote: > 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 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 <da...@gibson.dropbear.id.au> > --- > hw/ppc/spapr.c | 11 ++++++++--- > 1 file changed, 8 insertions(+), 3 deletions(-)
Reviewed-by: Laurent Vivier <lviv...@redhat.com>
