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[Qemu-devel] Re: [PATCH 2/6] qemu-kvm: Modify and introduce wrapper func


From: Avi Kivity
Subject: [Qemu-devel] Re: [PATCH 2/6] qemu-kvm: Modify and introduce wrapper functions to access phys_ram_dirty.
Date: Tue, 16 Mar 2010 14:45:44 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.8) Gecko/20100301 Fedora/3.0.3-1.fc12 Thunderbird/3.0.3

On 03/16/2010 12:53 PM, Yoshiaki Tamura wrote:
Modifies wrapper functions for byte-based phys_ram_dirty bitmap to
bit-based phys_ram_dirty bitmap, and adds more wrapper functions to prevent
direct access to the phys_ram_dirty bitmap.

+
+static inline int cpu_physical_memory_get_dirty_flags(ram_addr_t addr)
+{
+    unsigned long mask;
+    int index = (addr>>  TARGET_PAGE_BITS) / HOST_LONG_BITS;
+    int offset = (addr>>  TARGET_PAGE_BITS)&  (HOST_LONG_BITS - 1);
+    int ret = 0;
+
+    mask = 1UL<<  offset;
+    if (phys_ram_vga_dirty[index]&  mask)
+        ret |= VGA_DIRTY_FLAG;
+    if (phys_ram_code_dirty[index]&  mask)
+        ret |=  CODE_DIRTY_FLAG;
+    if (phys_ram_migration_dirty[index]&  mask)
+        ret |= MIGRATION_DIRTY_FLAG;
+
+    return ret;
  }

  static inline int cpu_physical_memory_get_dirty(ram_addr_t addr,
                                                  int dirty_flags)
  {
-    return phys_ram_dirty[addr>>  TARGET_PAGE_BITS]&  dirty_flags;
+    return cpu_physical_memory_get_dirty_flags(addr)&  dirty_flags;
  }

This turns one cacheline access into three. If the dirty bitmaps were in an array, you could do

return dirty_bitmaps[dirty_index][addr >> (TARGET_PAGE_BITS + BITS_IN_LONG)] & mask;

with one cacheline access.


  static inline void cpu_physical_memory_set_dirty(ram_addr_t addr)
  {
-    phys_ram_dirty[addr>>  TARGET_PAGE_BITS] = 0xff;
+    unsigned long mask;
+    int index = (addr>>  TARGET_PAGE_BITS) / HOST_LONG_BITS;
+    int offset = (addr>>  TARGET_PAGE_BITS)&  (HOST_LONG_BITS - 1);
+
+    mask = 1UL<<  offset;
+    phys_ram_vga_dirty[index] |= mask;
+    phys_ram_code_dirty[index] |= mask;
+    phys_ram_migration_dirty[index] |= mask;
+}

This is also three cacheline accesses. I think we should have a master bitmap which is updated by set_dirty(), and which is or'ed into the other bitmaps when they are accessed. At least the vga and migration bitmaps are only read periodically, not randomly, so this would be very fast. In a way, this is similar to how the qemu bitmap is updated from the kvm bitmap today.

I am not sure about the code bitmap though.


--
error compiling committee.c: too many arguments to function





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