[PATCH v5 0/4] powerpc/papr_scm: Workaround for failure of drc bind after kexec

Vaibhav Jain vaibhav at linux.ibm.com
Wed Jul 24 02:13:53 AEST 2019

Presently an error is returned in response to hcall H_SCM_BIND_MEM when a
new kernel boots on lpar via kexec. This prevents papr_scm from registering
drc memory regions with nvdimm. The error reported is of the form below:

"papr_scm ibm,persistent-memory:ibm,pmemory at 44100002: bind err: -68"

On investigation it was revealed that phyp returns this error as previous
kernel did not completely release bindings for drc scm-memory blocks and
hence phyp rejected request for re-binding these block to lpar with error
H_OVERLAP. Also support for a new H_SCM_UNBIND_ALL is recently added which
is better suited for releasing all the bound scm-memory block from an lpar.

So leveraging new hcall H_SCM_UNBIND_ALL, we can workaround H_OVERLAP issue
during kexec by forcing an unbind of all drm scm-memory blocks and issuing
H_SCM_BIND_MEM to re-bind the drc scm-memory blocks to lpar. This sequence
will also be needed when a new kernel boot on lpar after previous kernel
panicked and it never got an opportunity to call H_SCM_UNBIND_MEM/ALL.

Hence this patch-set implements following changes to papr_scm module:

* Update hvcall.h to include opcodes for new hcall H_SCM_UNBIND_ALL.

* Update it to use H_SCM_UNBIND_ALL instead of H_SCM_UNBIND_MEM

* In case hcall H_SCM_BIND_MEM fails with error H_OVERLAP, force
  H_SCM_UNBIND_ALL and retry the bind operation again.

With the patch-set applied re-bind of drc scm-memory to lpar succeeds after
a kexec to new kernel as illustrated below:

# Old kernel
$ sudo ndctl list -R
# kexec to new kernel
$ sudo kexec --initrd=... vmlinux
I'm in purgatory
papr_scm ibm,persistent-memory:ibm,pmemory at 44100002: Un-binding and retrying
# New kernel
$ sudo ndctl list -R

* Added a new doc-patch describing the HCALL interface between a guest kernel
  and PAPR compliant hyper-visor like PowerVM/KVM.

* Updated the patch description of first patch in the series as suggested
  by Mpe.

* Fixed a build warning reported by kbuild test robot.
* Updated the hcall opcode from latest papr-scm specification.
* Fixed a minor code comment & patch description as pointed out by Oliver.

* Addressed review comments from Oliver on v1 patchset.

Vaibhav Jain (4):
  powerpc: Document some HCalls for Storage Class Memory
  powerpc/pseries: Update SCM hcall op-codes in hvcall.h
  powerpc/papr_scm: Update drc_pmem_unbind() to use H_SCM_UNBIND_ALL
  powerpc/papr_scm: Force a scm-unbind if initial scm-bind fails

 Documentation/powerpc/hcalls.txt          | 140 ++++++++++++++++++++++
 arch/powerpc/include/asm/hvcall.h         |  11 +-
 arch/powerpc/platforms/pseries/papr_scm.c |  44 +++++--
 3 files changed, 184 insertions(+), 11 deletions(-)
 create mode 100644 Documentation/powerpc/hcalls.txt


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