Remove NX bit from x86-64 pages
PATCH 1/1 Remove NX bit from x86-64 pages
So after a sometime of looking at the code to solve bugzilla #75
http://bugzilla.xensource.com/cgi-bin/bugzilla/show_bug.cgi?id=75
I started to look for strange avenues of why this problem was happening. Well I found it...on IBM hardware with Intel EM64T cpus the ability for the CPU to understand the NX bit is disabled by default. I had no idea this was even possible. Now on AMD 64bit CPUs the CPU always understands the bit and the OS tells it to enforce it or not. Given this I'm betting that not just IBM hardware but hardware from other vendors may also ship with it turned off.
Our problem is that we assume that all x86-64 CPUs will understand this bit. We also do not have support fully implemented to take advantage of this bit. A patch from Scott Parish showed this by accident (his patch removed the NX bit from the pages)...which is how David Barrera the ability to boot it. So until full nx support is to go in we should remove it from from the pages for now too avoid strange issues as bugzilla #75....otherwise they will flood in.
The patch attached removes the bit:
include/asm-xen/asm-x86_64/pgtable.h renamed from include/asm-xen/asm-x86_64/pgtable.h.old +6 -6
@@ -191,21 +191,21 @@ static inline pte_t ptep_get_and_clear(p#define _PAGE_CHG_MASK (PTE_MASK | _PAGE_ACCESSED | _PAGE_DIRTY)#define PAGE_NONE __pgprot(_PAGE_PROTNONE | _PAGE_ACCESSED) -#define PAGE_SHARED __pgprot(_PAGE_PRESENT | _PAGE_RW | _PAGE_USER | _PAGE_ACCESSED | _PAGE_NX) +#define PAGE_SHARED __pgprot(_PAGE_PRESENT | _PAGE_RW | _PAGE_USER | _PAGE_ACCESSED)#define PAGE_SHARED_EXEC __pgprot(_PAGE_PRESENT | _PAGE_RW | _PAGE_USER | _PAGE_ACCESSED) -#define PAGE_COPY_NOEXEC __pgprot(_PAGE_PRESENT | _PAGE_USER | _PAGE_ACCESSED | _PAGE_NX) +#define PAGE_COPY_NOEXEC __pgprot(_PAGE_PRESENT | _PAGE_USER | _PAGE_ACCESSED)#define PAGE_COPY PAGE_COPY_NOEXEC#define PAGE_COPY_EXEC __pgprot(_PAGE_PRESENT | _PAGE_USER | _PAGE_ACCESSED) -#define PAGE_READONLY __pgprot(_PAGE_PRESENT | _PAGE_USER | _PAGE_ACCESSED | _PAGE_NX) +#define PAGE_READONLY __pgprot(_PAGE_PRESENT | _PAGE_USER | _PAGE_ACCESSED)#define PAGE_READONLY_EXEC __pgprot(_PAGE_PRESENT | _PAGE_USER | _PAGE_ACCESSED)#define __PAGE_KERNEL \ - (_PAGE_PRESENT | _PAGE_RW | _PAGE_DIRTY | _PAGE_ACCESSED | _PAGE_NX | _PAGE_USER ) + (_PAGE_PRESENT | _PAGE_RW | _PAGE_DIRTY | _PAGE_ACCESSED | _PAGE_USER )#define __PAGE_KERNEL_EXEC \ (_PAGE_PRESENT | _PAGE_RW | _PAGE_DIRTY | _PAGE_ACCESSED | _PAGE_USER )#define __PAGE_KERNEL_NOCACHE \ - (_PAGE_PRESENT | _PAGE_RW | _PAGE_DIRTY | _PAGE_PCD | _PAGE_ACCESSED | _PAGE_NX | _PAGE_USER ) + (_PAGE_PRESENT | _PAGE_RW | _PAGE_DIRTY | _PAGE_PCD | _PAGE_ACCESSED | _PAGE_USER )#define __PAGE_KERNEL_RO \ - (_PAGE_PRESENT | _PAGE_DIRTY | _PAGE_ACCESSED | _PAGE_NX | _PAGE_USER ) + (_PAGE_PRESENT | _PAGE_DIRTY | _PAGE_ACCESSED | _PAGE_USER )#define __PAGE_KERNEL_VSYSCALL \ (_PAGE_PRESENT | _PAGE_USER | _PAGE_ACCESSED | _PAGE_USER )#define __PAGE_KERNEL_VSYSCALL_NOCACHE \
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