x86: change _PAGE_GNTTAB

Since Linux started to use one of the 3 low available bits, _PAGE_IO
needed to be moved to a different one. Not remembering about
_PAGE_GNTTAB in debug hypervisors, I ended up assigning it to the same
bit, which made the kernel fail on the debug hypervisor. However,
rather than fixing the kernel it seems more appropriate for the
hypervisor to stay away from these bits, not the least because its
definition was anyway accompanied by a warning that this may be
incompatible with certain OSes.

While obviously the hypervisor has to use some bit (and it's therefore
unavoidable that there's some risk of collision), using one of the
high available bits seems to be the better choice over using one of
the three low ones. Since in 32-bit mode these bits are reserved, the
patch disables the functionality here. The only reasonable alternative
I would see is to disable the functionality by default, but add a
command line option to specify which bit to use.

Signed-off-by: Jan Beulich <jbeulich@novell.com>

diff --git a/xen/include/asm-x86/page.h b/xen/include/asm-x86/page.h
index 3b134eb5d7c61ac2d27c25805d91d9c5415adabd..c6b2e53d21d7a569e48438d555936b2931be1346 100644 (file)
--- a/xen/include/asm-x86/page.h
+++ b/xen/include/asm-x86/page.h
@@ -302,9 +302,10 @@ unsigned long clone_idle_pagetable(struct vcpu *);
  * WARNING: This will need to be disabled to run OSes that use the spare PTE
  * bits themselves (e.g., *BSD).
  */
-#ifndef NDEBUG
-#define _PAGE_GNTTAB   _PAGE_AVAIL2
-#else
+#ifdef NDEBUG
+#undef _PAGE_GNTTAB
+#endif
+#ifndef _PAGE_GNTTAB
 #define _PAGE_GNTTAB   0
 #endif
 

diff --git a/xen/include/asm-x86/x86_64/page.h b/xen/include/asm-x86/x86_64/page.h
index e0223f9a297826314336ff9060f1e60ea01c7623..f20f053dd51f4630b981be67702c7711efc486ac 100644 (file)
--- a/xen/include/asm-x86/x86_64/page.h
+++ b/xen/include/asm-x86/x86_64/page.h
@@ -104,6 +104,9 @@ typedef l4_pgentry_t root_pgentry_t;
 #define _PAGE_NX_BIT (1U<<23)
 #define _PAGE_NX     (cpu_has_nx ? _PAGE_NX_BIT : 0U)
 
+/* Bit 22 of a 24-bit flag mask. This corresponds to bit 62 of a pte.*/
+#define _PAGE_GNTTAB (1U<<22)
+
 /*
  * Disallow unused flag bits plus PAT/PSE, PCD, PWT and GLOBAL.
  * Permit the NX bit if the hardware supports it.