[PATCH 16/16] xen/i386: make sure initial VGA/ISA mappings are not overridden

[Jeremy Fitzhardinge <jeremy@goop.org>]

From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/mm/init_32.c overrides the ISA/VGA mappings with direct mappings
which do not have _PAGE_IOMAP set, thereby making the ISA space inaccessible.

This patch adds to the existing hack to make sure the pre-constructed
ISA mappings are not incorrectly overwritten.

Thanks to Gerd Hoffman for pointing this out.

[ Impact: Makes 32-bit dom0 VGA work properly. ]

Diagnosed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
---
 arch/x86/xen/mmu.c |   11 +++++++++--
 1 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 85d9d18..331e52d 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1502,10 +1502,17 @@ static void *xen_kmap_atomic_pte(struct page *page, enum km_type type)
 #ifdef CONFIG_X86_32
 static __init pte_t mask_rw_pte(pte_t *ptep, pte_t pte)
 {
-	/* If there's an existing pte, then don't allow _PAGE_RW to be set */
-	if (pte_val_ma(*ptep) & _PAGE_PRESENT)
+	pte_t oldpte = *ptep;
+
+	if (pte_flags(oldpte) & _PAGE_PRESENT) {
+		/* Don't allow existing IO mappings to be overridden */
+		if (pte_flags(oldpte) & _PAGE_IOMAP)
+			pte = oldpte;
+
+		/* Don't allow _PAGE_RW to be set on existing pte */
 		pte = __pte_ma(((pte_val_ma(*ptep) & _PAGE_RW) | ~_PAGE_RW) &
 			       pte_val_ma(pte));
+	}
 
 	return pte;
 }
-- 
1.6.0.6