On Fri, Jul 24, 2020 at 01:26:00PM +0530, Aneesh Kumar K.V wrote:

This adds a kernel command line option that can be used to disable GTSE support.
Disabling GTSE implies kernel will make hcalls to invalidate TLB entries.

This was done so that we can do VM migration between configs that enable/disable
GTSE support via hypervisor. To migrate a VM from a system that supports
GTSE to a system that doesn't, we can boot the guest with radix_gtse=off, thereby
forcing the guest to use hcalls for TLB invalidates.

The check for hcall availability is done in pSeries_setup_arch so that
the panic message appears on the console. This should only happen on
a hypervisor that doesn't force the guest to hash translation even
though it can't handle the radix GTSE=0 request via CAS. With radix_gtse=off
if the hypervisor doesn't support hcall_rpt_invalidate hcall it should
force the LPAR to hash translation.

Signed-off-by: Aneesh Kumar K.V <redacted>
---
 Documentation/admin-guide/kernel-parameters.txt |  3 +++
 arch/powerpc/include/asm/firmware.h             |  4 +++-
 arch/powerpc/kernel/prom_init.c                 | 13 +++++++++----
 arch/powerpc/platforms/pseries/firmware.c       |  1 +
 arch/powerpc/platforms/pseries/setup.c          |  5 +++++
 5 files changed, 21 insertions(+), 5 deletions(-)

 
Tested

1. radix_gtse=off with KVM implementation of H_RPT_INVALIDATE hcall, the
   tlb flush calls get off-loaded to hcalls.
2. radix_gtse=off w/o H_RPT_INVALIDATE hcall, the guest kernel panics
   as per design.

However in both cases, the guest kernel prints out
"WARNING: Hypervisor doesn't support RADIX with GTSE" which can be a bit
confusing in case 1 as GTSE has disabled by the guest and hypervisor is
capable of supporting the same via hcall.

Regards,
Bharata.