arm64: Add support for hardware updates of the access and dirty pte bits
The ARMv8.1 architecture extensions introduce support for hardware
updates of the access and dirty information in page table entries. With
TCR_EL1.HA enabled, when the CPU accesses an address with the PTE_AF bit
cleared in the page table, instead of raising an access flag fault the
CPU sets the actual page table entry bit. To ensure that kernel
modifications to the page tables do not inadvertently revert a change
introduced by hardware updates, the exclusive monitor (ldxr/stxr) is
adopted in the pte accessors.
When TCR_EL1.HD is enabled, a write access to a memory location with the
DBM (Dirty Bit Management) bit set in the corresponding pte
automatically clears the read-only bit (AP[2]). Such DBM bit maps onto
the Linux PTE_WRITE bit and to check whether a writable (DBM set) page
is dirty, the kernel tests the PTE_RDONLY bit. In order to allow
read-only and dirty pages, the kernel needs to preserve the software
dirty bit. The hardware dirty status is transferred to the software
dirty bit in ptep_set_wrprotect() (using load/store exclusive loop) and
pte_modify().
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index 39139a3..a8be513 100644
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -196,6 +196,19 @@
*/
mrs x9, ID_AA64MMFR0_EL1
bfi x10, x9, #32, #3
+#ifdef CONFIG_ARM64_HW_AFDBM
+ /*
+ * Hardware update of the Access and Dirty bits.
+ */
+ mrs x9, ID_AA64MMFR1_EL1
+ and x9, x9, #0xf
+ cbz x9, 2f
+ cmp x9, #2
+ b.lt 1f
+ orr x10, x10, #TCR_HD // hardware Dirty flag update
+1: orr x10, x10, #TCR_HA // hardware Access flag update
+2:
+#endif /* CONFIG_ARM64_HW_AFDBM */
msr tcr_el1, x10
ret // return to head.S
ENDPROC(__cpu_setup)