[PATCH for-6.0 v2 0/3] spapr: Address the confusion between IPI numbers and vCPU ids

[Greg Kurz]

A regression was recently fixed in the sPAPR XIVE code for QEMU 5.2
RC3 [1]. It boiled down to a confusion between IPI numbers and vCPU
ids, which happen to be numerically equal in general, but are really
different entities that can diverge in some setups. This was causing
QEMU to misconfigure XIVE and to crash the guest.

The confusion comes from XICS actually. Interrupt presenters in XICS
are identified by a "server number" which is a 1:1 mapping to vCPU
ids. The range of these "server numbers" is exposed to the guest in
the "ibm,interrupt-server-ranges" property. A xics_max_server_number()
helper was introduced at some point to compute the upper limit of the
range. When XIVE was added, commit 1a518e7693c9 renamed the helper to
spapr_max_server_number(). It ended up being used to size a bunch of
things in XIVE that are per-vCPU, such as internal END tables or
IPI ranges presented to the guest. The problem is that the maximum
"server number" can be much higher (up to 8 times) than the actual
number of vCPUs when the VSMT mode doesn't match the number of threads
per core in the guest:

    DIV_ROUND_UP(ms->smp.max_cpus * spapr->vsmt, ms->smp.threads);

Since QEMU 4.2, the default behavior is to set spapr->vsmt to
ms->smp.threads. Setups with custom VSMT settings will configure XIVE
to use more HW resources than needed. This is a bit unfortunate but
not extremely harmful, unless maybe if a lot of guests are running
on the host. The sizing of the IPI range is more problematic though
as it eventually led to [1].

This series first does some renaming to make it clear when we're
dealing with vCPU ids. It then fixes the machine code to pass
smp.max_cpus to XIVE where appropriate. Since these changes are
guest/migration visible, a machine property is added to keep the
existing behavior for older machine types. The series is thus based
on Connie's recent patch that introduces compat machines for
QEMU 6.0.

Based-on: 20201109173928.1001764-1-cohuck@redhat.com

Note that we still use spapr_max_vcpu_ids() when activating the
in-kernel irqchip because this is what both XICS-on-XIVE and XIVE
KVM devices expect.

[1] https://bugs.launchpad.net/qemu/+bug/1900241

v2: - comments on v1 highlighted that problems mostly come from
      spapr_max_server_number() which got misused over the years.
      Updated the cover letter accordingly.
    - completely new approach. Instead of messing with device properties,
      pass the appropriate values to the IC backend handlers.
    - rename a few things using the "max_vcpu_ids" wording instead of
      "nr_servers" and "max_server_number"

Greg Kurz (3):
  spapr: Improve naming of some vCPU id related items
  spapr/xive: Fix size of END table and number of claimed IPIs
  spapr/xive: Fix the "ibm,xive-lisn-ranges" property

 include/hw/ppc/spapr.h      |  3 ++-
 include/hw/ppc/spapr_irq.h  | 12 ++++++------
 include/hw/ppc/spapr_xive.h |  2 +-
 include/hw/ppc/xics_spapr.h |  2 +-
 hw/intc/spapr_xive.c        |  9 +++++----
 hw/intc/spapr_xive_kvm.c    |  4 ++--
 hw/intc/xics_kvm.c          |  4 ++--
 hw/intc/xics_spapr.c        | 11 ++++++-----
 hw/ppc/spapr.c              | 12 ++++++++----
 hw/ppc/spapr_irq.c          | 34 ++++++++++++++++++++++++----------
 10 files changed, 57 insertions(+), 36 deletions(-)

-- 
2.26.2