On 06/01/2018 01:47, Jose Ricardo Ziviani wrote: > Power9 supports 4 HW threads/core but it's possible to emulate > doorbells to implement virtual SMT. KVM has the KVM_CAP_PPC_SMT_POSSIBLE > which returns a bitmap with all SMT modes supported by the host. > > Today, QEMU forces the SMT mode based on PVR compat table, this is > silently done in spapr_fixup_cpu_dt. Then, if user passes thread=8 the > guest will end up with 4 threads/core without any feedback to the user. > It is confusing and will crash QEMU if a cpu is hotplugged in that > guest. > > This patch makes use of KVM_CAP_PPC_SMT_POSSIBLE to check if the host > supports the SMT mode so it allows Power9 guests to have 8 threads/core > if desired. > > Reported-by: Satheesh Rajendran <[email protected]> > Signed-off-by: Jose Ricardo Ziviani <[email protected]> > --- > hw/ppc/spapr.c | 14 +++++++++++++- > hw/ppc/trace-events | 1 + > target/ppc/kvm.c | 5 +++++ > target/ppc/kvm_ppc.h | 6 ++++++ > 4 files changed, 25 insertions(+), 1 deletion(-)
According to the tests I have done on P9 and P8, you should also set vsmt to the max value found in KVM_CAP_PPC_SMT_POSSIBLE to keep the same vcpu_id on a P8 and P9 with the same configuration "-smp W,sockets=X,cores=Y,threads=Z". This is required for migration. The formula is in spapr_cpu_core_realize(): cpu->vcpu_id = (cc->core_id * spapr->vsmt / smp_threads) + i); Thanks, Laurent
