Using 64k pages on 64-bit PowerPC systems makes life difficult for
emulators that are trying to emulate an ISA, such as x86, which use a
smaller page size, since the emulator can no longer use the MMU and
the normal system calls for controlling page protections. Of course,
the emulator can emulate the MMU by checking and possibly remapping
the address for each memory access in software, but that is pretty
slow.
This provides a facility for such programs to control the access
permissions on individual 4k sub-pages of 64k pages. The idea is
that the emulator supplies an array of protection masks to apply to a
specified range of virtual addresses. These masks are applied at the
level where hardware PTEs are inserted into the hardware page table
based on the Linux PTEs, so the Linux PTEs are not affected. Note
that this new mechanism does not allow any access that would otherwise
be prohibited; it can only prohibit accesses that would otherwise be
allowed. This new facility is only available on 64-bit PowerPC and
only when the kernel is configured for 64k pages.
The masks are supplied using a new subpage_prot system call, which
takes a starting virtual address and length, and a pointer to an array
of protection masks in memory. The array has a 32-bit word per 64k
page to be protected; each 32-bit word consists of 16 2-bit fields,
for which 0 allows any access (that is otherwise allowed), 1 prevents
write accesses, and 2 or 3 prevent any access.
Implicit in this is that the regions of the address space that are
protected are switched to use 4k hardware pages rather than 64k
hardware pages (on machines with hardware 64k page support). In fact
the whole process is switched to use 4k hardware pages when the
subpage_prot system call is used, but this could be improved in future
to switch only the affected segments.
The subpage protection bits are stored in a 3 level tree akin to the
page table tree. The top level of this tree is stored in a structure
that is appended to the top level of the page table tree, i.e., the
pgd array. Since it will often only be 32-bit addresses (below 4GB)
that are protected, the pointers to the first four bottom level pages
are also stored in this structure (each bottom level page contains the
protection bits for 1GB of address space), so the protection bits for
addresses below 4GB can be accessed with one fewer loads than those
for higher addresses.
Signed-off-by: Paul Mackerras <[EMAIL PROTECTED]>
---
This version allocates a new system call number for the subpage_prot
syscall.
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 66a3d8c..53d0955 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -342,6 +342,14 @@ config PPC_64K_PAGES
while on hardware with such support, it will be used to map
normal application pages.
+config PPC_SUBPAGE_PROT
+ bool "Support setting protections for 4k subpages"
+ depends on PPC_64K_PAGES
+ help
+ This option adds support for a system call to allow user programs
+ to set access permissions (read/write, readonly, or no access)
+ on the 4k subpages of each 64k page.
+
config SCHED_SMT
bool "SMT (Hyperthreading) scheduler support"
depends on PPC64 && SMP
diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index c349868..11b4f6d 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/arch/powerpc/kernel/head_64.S
@@ -903,6 +903,7 @@ handle_page_fault:
* the PTE insertion
*/
12:bl .save_nvgprs
+ mr r5,r3
addir3,r1,STACK_FRAME_OVERHEAD
ld r4,_DAR(r1)
bl .low_hash_fault
diff --git a/arch/powerpc/kernel/syscalls.c b/arch/powerpc/kernel/syscalls.c
index 3b1d5dd..18be22e 100644
--- a/arch/powerpc/kernel/syscalls.c
+++ b/arch/powerpc/kernel/syscalls.c
@@ -328,3 +328,7 @@ void do_show_syscall_exit(unsigned long r3)
{
printk(" -> %lx, current=%p cpu=%d\n", r3, current, smp_processor_id());
}
+
+#ifndef CONFIG_PPC_SUBPAGE_PROT
+cond_syscall(subpage_prot);
+#endif
diff --git a/arch/powerpc/mm/Makefile b/arch/powerpc/mm/Makefile
index 20629ae..41649a5 100644
--- a/arch/powerpc/mm/Makefile
+++ b/arch/powerpc/mm/Makefile
@@ -22,3 +22,4 @@ obj-$(CONFIG_FSL_BOOKE) += fsl_booke_mmu.o
obj-$(CONFIG_NEED_MULTIPLE_NODES) += numa.o
obj-$(CONFIG_PPC_MM_SLICES)+= slice.o
obj-$(CONFIG_HUGETLB_PAGE) += hugetlbpage.o
+obj-$(CONFIG_PPC_SUBPAGE_PROT) += subpage-prot.o
diff --git a/arch/powerpc/mm/hash_low_64.S b/arch/powerpc/mm/hash_low_64.S
index e935edd..21d2484 100644
--- a/arch/powerpc/mm/hash_low_64.S
+++ b/arch/powerpc/mm/hash_low_64.S
@@ -331,7 +331,8 @@ htab_pte_insert_failure:
*/
/* _hash_page_4K(unsigned long ea, unsigned long access, unsigned long vsid,
- * pte_t *ptep, unsigned long trap, int local, int ssize)
+ * pte_t *ptep, unsigned lo
