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

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

[Paolo Bonzini]

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>
Tested-by: Anthony Perard <anthony.perard@citrix.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 exec.c | 33 ++++++++++++++++++---------------
 1 file changed, 18 insertions(+), 15 deletions(-)

diff --git a/exec.c b/exec.c
index 9ad0a4b..ecfda77 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)
@@ -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

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

[Mark Cave-Ayland]

On 06/02/14 23:40, Paolo Bonzini wrote:

> 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>
> Tested-by: Anthony Perard<anthony.perard@citrix.com>
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Paolo Bonzini<pbonzini@redhat.com>

Hi Paolo,

Unfortunately I've just tested this patch on qemu-system-ppc and it 
still doesn't fix the issue for me :(  This is with current git master 
bc1c7 plus your patch applied.

The symptoms haven't changed in that the VGA output stays black and 
doesn't resize, again implying that the VGA VBE calls aren't working. If 
it helps, the OpenBIOS VGA driver vga.fs can be found at 
http://git.qemu.org/?p=openbios.git;a=blob;f=drivers/vga.fs;h=ec4c6c5f10ea2c4c784c3c6888c3805ec3b14496;hb=888126272f92294b0da45158393f1b862742cf6b. 



ATB,

Mark.