Re: [Qemu-devel] [Qemu-ppc] [PATCH 1/1] spapr: Check SMT based on KVM_CAP_PPC_SMT_POSSIBLE

[Laurent Vivier <lvivier@redhat.com>]

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 <satheera@in.ibm.com>
> Signed-off-by: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
> ---
>  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