Re: [Qemu-devel] [PATCH v2 00/39] bitmap handling optimization

[Paolo Bonzini]

Il 06/11/2013 15:37, Gerd Hoffmann ha scritto:
> 
>>> - vga ram by default is not aligned in a page number multiple of
>>> 64,
>>> 
>>> it could be optimized.  Kraxel?  It syncs the kvm bitmap at least
>>> 1 a second or so? bitmap is only 2048 pages (16MB by default). We
>>> need to change the ram_addr only
> It is created using memory_region_init_ram(), in vga_common_init(). 
> Nothing special is done to avoid/force any alignment. Dunno why it
> ends up on a odd page number.

Because some random option ROM is loaded before the RAM region is
created, and thus the ram_addr_t's become misaligned.  Keeping the
ram_addr_t's aligned in find_ram_offset is easy:

diff --git a/exec.c b/exec.c
index 79610ce..1b82e81 100644
--- a/exec.c
+++ b/exec.c
@@ -1002,7 +1002,7 @@ static ram_addr_t find_ram_offset(ram_addr_t size)
     QTAILQ_FOREACH(block, &ram_list.blocks, next) {
         ram_addr_t end, next = RAM_ADDR_MAX;
 
-        end = block->offset + block->length;
+        end = ROUND_UP(block->offset + block->length, TARGET_PAGE_SIZE * 64);
 
         QTAILQ_FOREACH(next_block, &ram_list.blocks, next) {
             if (next_block->offset >= end) {

but I'm not sure if we're allowed to change the ram_addr_t's.  On one hand they
are not part of guest ABI, on the other hand RDMA migration uses them
in the protocol.

Michael, can you check if RDMA migration works from a QEMU *without* 
this patch to one *with*:

diff --git a/exec.c b/exec.c
index 79610ce..21b8b96 100644
--- a/exec.c
+++ b/exec.c
@@ -997,7 +997,7 @@ static ram_addr_t find_ram_offset(ram_addr_t size)
     assert(size != 0); /* it would hand out same offset multiple times */
 
     if (QTAILQ_EMPTY(&ram_list.blocks))
-        return 0;
+        return TARGET_PAGE_SIZE * 100;
 
     QTAILQ_FOREACH(block, &ram_list.blocks, next) {
         ram_addr_t end, next = RAM_ADDR_MAX;

I suspect it doesn't.  But perhaps we still have time before RDMA
becomes non-experimental.

Paolo