Date: Wed, 19 Jul 2006 14:11:40 -0500 From: Alan Cox <alc@cs.rice.edu> To: arch@freebsd.org Subject: On the coming demise of debug.mpsafevm and pmap_page_protect() Message-ID: <44BE83EC.9060104@cs.rice.edu>
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In the coming days, I am planning on removing debug.mpsafevm and pmap_page_protect() from the virtual memory system. As for debug.mpsafevm, all of the architectures in CVS have it enabled by default, some for over two years. The only impact should be on the nascent MIPS port. As for pmap_page_protect(), the proposed commit message follows. 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 source tree 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@
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