[Qemu-devel] [PATCH] Use Little Endian for Dirty Log

[Alexander Graf <agraf@suse.de>]

We currently use host endian long types to store information
in the dirty bitmap.

This works reasonably well on Little Endian targets, because the
u32 after the first contains the next 32 bits. On Big Endian this
breaks completely though, forcing us to be inventive here.

So Ben suggested to always use Little Endian, which looks reasonable.

We only have dirty bitmap implemented in Little Endian targets so far
and since PowerPC would be the first Big Endian platform, we can just
as well switch to Little Endian always with little effort without
breaking existing targets.

This is the userspace part of the patch. It shouldn't change anything
for existing targets, but help PowerPC.

It replaces my older patch called "Use 64bit pointer for dirty log".

Signed-off-by: Alexander Graf <agraf@suse.de>
---
 kvm-all.c |   11 +++++++----
 1 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/kvm-all.c b/kvm-all.c
index 824bb4c..91d9333 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -304,6 +304,11 @@ int kvm_set_migration_log(int enable)
     return 0;
 }
 
+static int test_le_bit(unsigned long nr, unsigned char *addr)
+{
+    return (addr[nr >> 3] >> (nr & 7)) & 1;
+}
+
 /**
  * kvm_physical_sync_dirty_bitmap - Grab dirty bitmap from kernel space
  * This function updates qemu's dirty bitmap using cpu_physical_memory_set_dirty().
@@ -357,12 +362,10 @@ int kvm_physical_sync_dirty_bitmap(target_phys_addr_t start_addr,
         for (phys_addr = mem->start_addr, addr = mem->phys_offset;
              phys_addr < mem->start_addr + mem->memory_size;
              phys_addr += TARGET_PAGE_SIZE, addr += TARGET_PAGE_SIZE) {
-            unsigned long *bitmap = (unsigned long *)d.dirty_bitmap;
+            unsigned char *bitmap = (unsigned char *)d.dirty_bitmap;
             unsigned nr = (phys_addr - mem->start_addr) >> TARGET_PAGE_BITS;
-            unsigned word = nr / (sizeof(*bitmap) * 8);
-            unsigned bit = nr % (sizeof(*bitmap) * 8);
 
-            if ((bitmap[word] >> bit) & 1) {
+            if (test_le_bit(nr, bitmap)) {
                 cpu_physical_memory_set_dirty(addr);
             } else if (r < 0) {
                 /* When our KVM implementation doesn't know about dirty logging
-- 
1.6.0.2