s390/mm: add support for RDP (Reset DAT-Protection)

RDP instruction allows to reset DAT-protection bit in a PTE, with less
CPU synchronization overhead than IPTE instruction. In particular, IPTE
can cause machine-wide synchronization overhead, and excessive IPTE usage
can negatively impact machine performance.

RDP can be used instead of IPTE, if the new PTE only differs in SW bits
and _PAGE_PROTECT HW bit, for PTE protection changes from RO to RW.
SW PTE bit changes are allowed, e.g. for dirty and young tracking, but none
of the other HW-defined part of the PTE must change. This is because the
architecture forbids such changes to an active and valid PTE, which
is why invalidation with IPTE is always used first, before writing a new
entry.

The RDP optimization helps mainly for fault-driven SW dirty-bit tracking.
Writable PTEs are initially always mapped with HW _PAGE_PROTECT bit set,
to allow SW dirty-bit accounting on first write protection fault, where
the DAT-protection would then be reset. The reset is now done with RDP
instead of IPTE, if RDP instruction is available.

RDP cannot always guarantee that the DAT-protection reset is propagated
to all CPUs immediately. This means that spurious TLB protection faults
on other CPUs can now occur. For this, common code provides a
flush_tlb_fix_spurious_fault() handler, which will now be used to do a
CPU-local TLB flush. However, this will clear the whole TLB of a CPU, and
not just the affected entry. For more fine-grained flushing, by simply
doing a (local) RDP again, flush_tlb_fix_spurious_fault() would need to
also provide the PTE pointer.

Note that spurious TLB protection faults cannot really be distinguished
from racing pagetable updates, where another thread already installed the
correct PTE. In such a case, the local TLB flush would be unnecessary
overhead, but overall reduction of CPU synchronization overhead by not
using IPTE is still expected to be beneficial.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

1 files changed, 25 insertions, 0 deletions

diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
index 4909dcd762e8..6effb24de6d9 100644
--- a/arch/s390/mm/pgtable.c
+++ b/arch/s390/mm/pgtable.c
@@ -302,6 +302,31 @@ pte_t ptep_xchg_direct(struct mm_struct *mm, unsigned long addr,
 }
 EXPORT_SYMBOL(ptep_xchg_direct);
 
+/*
+ * Caller must check that new PTE only differs in _PAGE_PROTECT HW bit, so that
+ * RDP can be used instead of IPTE. See also comments at pte_allow_rdp().
+ */
+void ptep_reset_dat_prot(struct mm_struct *mm, unsigned long addr, pte_t *ptep,
+			 pte_t new)
+{
+	preempt_disable();
+	atomic_inc(&mm->context.flush_count);
+	if (cpumask_equal(mm_cpumask(mm), cpumask_of(smp_processor_id())))
+		__ptep_rdp(addr, ptep, 0, 0, 1);
+	else
+		__ptep_rdp(addr, ptep, 0, 0, 0);
+	/*
+	 * PTE is not invalidated by RDP, only _PAGE_PROTECT is cleared. That
+	 * means it is still valid and active, and must not be changed according
+	 * to the architecture. But writing a new value that only differs in SW
+	 * bits is allowed.
+	 */
+	set_pte(ptep, new);
+	atomic_dec(&mm->context.flush_count);
+	preempt_enable();
+}
+EXPORT_SYMBOL(ptep_reset_dat_prot);
+
 pte_t ptep_xchg_lazy(struct mm_struct *mm, unsigned long addr,
 		     pte_t *ptep, pte_t new)
 {