powerpc/64s/radix: do not flush TLB on spurious fault

powerpc/64s/radix: do not flush TLB on spurious fault

In the case of a spurious fault (which can happen due to a race with
another thread that changes the page table), the default Linux mm code
calls flush_tlb_page for that address. This is not required because
the pte will be re-fetched. Hash does not wire this up to a hardware
TLB flush for this reason. This patch avoids the flush for radix.

>From Power ISA v3.0B, p.1090:

    Setting a Reference or Change Bit or Upgrading Access Authority
    (PTE Subject to Atomic Hardware Updates)

    If the only change being made to a valid PTE that is subject to
    atomic hardware updates is to set the Refer- ence or Change bit to
    1 or to add access authorities, a simpler sequence suffices
    because the translation hardware will refetch the PTE if an access
    is attempted for which the only problems were reference and/or
    change bits needing to be set or insufficient access authority.

The nest MMU on POWER9 does not re-fetch the PTE after such an access
attempt before faulting, so address spaces with a coprocessor
attached will continue to flush in these cases.

This reduces tlbies for a kernel compile workload from 0.95M to 0.90M.

fork --fork --exec benchmark improved 0.5% (12300->12400).

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>