[Qemu-devel] [PATCH v2] memory: fix limiting of translation at a page boundary

[Paolo Bonzini <pbonzini@redhat.com>]

Commit 360e607 (address_space_translate: do not cross page boundaries,
2014-01-30) broke MMIO accesses in cases where the section is shorter
than the full register width.  This can happen for example with the
Bochs DISPI registers, which are 16 bits wide but have only a 1-byte
long MemoryRegion (if you write to the "second byte" of the register
your access is discarded; it doesn't write only to half of the register).

Restrict the action of commit 360e607 to direct RAM accesses.  This
is enough for Xen, since MMIO will not go through the mapcache.

Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
        v1->v2: also revert the change in the call to
        address_space_translate_internal.  Removed Anthony's
        Tested-by to be safe.

 exec.c | 35 +++++++++++++++++++----------------
 1 file changed, 19 insertions(+), 16 deletions(-)

diff --git a/exec.c b/exec.c
index 9ad0a4b..be93ebb 100644
--- a/exec.c
+++ b/exec.c
@@ -325,7 +325,7 @@ address_space_translate_internal(AddressSpaceDispatch *d, hwaddr addr, hwaddr *x
                                  hwaddr *plen, bool resolve_subpage)
 {
     MemoryRegionSection *section;
-    Int128 diff, diff_page;
+    Int128 diff;
 
     section = address_space_lookup_region(d, addr, resolve_subpage);
     /* Compute offset within MemoryRegionSection */
@@ -334,13 +334,23 @@ address_space_translate_internal(AddressSpaceDispatch *d, hwaddr addr, hwaddr *x
     /* Compute offset within MemoryRegion */
     *xlat = addr + section->offset_within_region;
 
-    diff_page = int128_make64(((addr & TARGET_PAGE_MASK) + TARGET_PAGE_SIZE) - addr);
     diff = int128_sub(section->mr->size, int128_make64(addr));
-    diff = int128_min(diff, diff_page);
     *plen = int128_get64(int128_min(diff, int128_make64(*plen)));
     return section;
 }
 
+static inline bool memory_access_is_direct(MemoryRegion *mr, bool is_write)
+{
+    if (memory_region_is_ram(mr)) {
+        return !(is_write && mr->readonly);
+    }
+    if (memory_region_is_romd(mr)) {
+        return !is_write;
+    }
+
+    return false;
+}
+
 MemoryRegion *address_space_translate(AddressSpace *as, hwaddr addr,
                                       hwaddr *xlat, hwaddr *plen,
                                       bool is_write)
@@ -351,7 +361,7 @@ MemoryRegion *address_space_translate(AddressSpace *as, hwaddr addr,
     hwaddr len = *plen;
 
     for (;;) {
-        section = address_space_translate_internal(as->dispatch, addr, &addr, &len, true);
+        section = address_space_translate_internal(as->dispatch, addr, &addr, plen, true);
         mr = section->mr;
 
         if (!mr->iommu_ops) {
@@ -370,6 +380,11 @@ MemoryRegion *address_space_translate(AddressSpace *as, hwaddr addr,
         as = iotlb.target_as;
     }
 
+    if (memory_access_is_direct(mr, is_write)) {
+        hwaddr page = ((addr & TARGET_PAGE_MASK) + TARGET_PAGE_SIZE) - addr;
+        len = MIN(page, len);
+    }
+
     *plen = len;
     *xlat = addr;
     return mr;
@@ -1925,18 +1940,6 @@ static void invalidate_and_set_dirty(hwaddr addr,
     xen_modified_memory(addr, length);
 }
 
-static inline bool memory_access_is_direct(MemoryRegion *mr, bool is_write)
-{
-    if (memory_region_is_ram(mr)) {
-        return !(is_write && mr->readonly);
-    }
-    if (memory_region_is_romd(mr)) {
-        return !is_write;
-    }
-
-    return false;
-}
-
 static int memory_access_size(MemoryRegion *mr, unsigned l, hwaddr addr)
 {
     unsigned access_size_max = mr->ops->valid.max_access_size;
-- 
1.8.5.3