Add pmap_clear_write() to the interface between the virtual memory

system's machine-dependent and machine-independent layers.  Once
pmap_clear_write() is implemented on all of our supported
architectures, I intend to replace all calls to pmap_page_protect() by
calls to pmap_clear_write().  Why?  Both the use and implementation of
pmap_page_protect() in our virtual memory system has subtle errors,
specifically, the management of execute permission is broken on some
architectures.  The "prot" argument to pmap_page_protect() should
behave differently from the "prot" argument to other pmap functions.
Instead of meaning, "give the specified access rights to all of the
physical page's mappings," it means "don't take away the specified
access rights from all of the physical page's mappings, but do take
away the ones that aren't specified."  However, owing to our i386
legacy, i.e., no support for no-execute rights, all but one invocation
of pmap_page_protect() specifies VM_PROT_READ only, when the intent
is, in fact, to remove only write permission.  Consequently, a
faithful implementation of pmap_page_protect(), e.g., ia64, would
remove execute permission as well as write permission.  On the other
hand, some architectures that support execute permission have
basically ignored whether or not VM_PROT_EXECUTE is passed to
pmap_page_protect(), e.g., amd64 and sparc64.  This change represents
the first step in replacing pmap_page_protect() by the less subtle
pmap_clear_write() that is already implemented on amd64, i386, and
sparc64.

Discussed with: grehan@ and marcel@

1 files changed, 1 insertions, 0 deletions

diff --git a/sys/vm/pmap.h b/sys/vm/pmap.h
index 57d62aa..8c1f9a3 100644
--- a/sys/vm/pmap.h
+++ b/sys/vm/pmap.h
@@ -93,6 +93,7 @@ extern vm_offset_t kernel_vm_end;
 void		 pmap_change_wiring(pmap_t, vm_offset_t, boolean_t);
 void		 pmap_clear_modify(vm_page_t m);
 void		 pmap_clear_reference(vm_page_t m);
+void		 pmap_clear_write(vm_page_t m);
 void		 pmap_copy(pmap_t, pmap_t, vm_offset_t, vm_size_t, vm_offset_t);
 void		 pmap_copy_page(vm_page_t, vm_page_t);
 void		 pmap_enter(pmap_t, vm_offset_t, vm_page_t, vm_prot_t,