Re: [Qemu-devel] [PATCH v2 3/3] CPUPhysMemoryClient: Batch contiguous addresses when playing catchup

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, May 03, 2011 at 12:36:58PM -0600, Alex Williamson wrote:
> When a phys memory client registers and we play catchup by walking
> the page tables, we can make a huge improvement in the number of
> times the set_memory callback is called by batching contiguous
> pages together.  With a 4G guest, this reduces the number of callbacks
> at registration from 1048866 to 296.
> 
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
> 
>  exec.c |   38 ++++++++++++++++++++++++++++++++------
>  1 files changed, 32 insertions(+), 6 deletions(-)
> 
> diff --git a/exec.c b/exec.c
> index bbd5c86..a0678a4 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -1741,14 +1741,21 @@ static int cpu_notify_migration_log(int enable)
>      return 0;
>  }
>  
> +struct last_map {
> +    target_phys_addr_t start_addr;
> +    ram_addr_t size;

A bit worried that ram_addr_t size might thinkably overflow
(it's just a long, could be a 4G ram). Break it out when it fills up?

> +    ram_addr_t phys_offset;
> +};
> +
>  /* The l1_phys_map provides the upper P_L1_BITs of the guest physical
>   * address.  Each intermediate table provides the next L2_BITs of guest
>   * physical address space.  The number of levels vary based on host and
>   * guest configuration, making it efficient to build the final guest
>   * physical address by seeding the L1 offset and shifting and adding in
>   * each L2 offset as we recurse through them. */
> -static void phys_page_for_each_1(CPUPhysMemoryClient *client,
> -                                 int level, void **lp, target_phys_addr_t addr)
> +static void phys_page_for_each_1(CPUPhysMemoryClient *client, int level,
> +                                 void **lp, target_phys_addr_t addr,
> +                                 struct last_map *map)
>  {
>      int i;
>  
> @@ -1760,15 +1767,29 @@ static void phys_page_for_each_1(CPUPhysMemoryClient *client,
>          addr <<= L2_BITS + TARGET_PAGE_BITS;
>          for (i = 0; i < L2_SIZE; ++i) {
>              if (pd[i].phys_offset != IO_MEM_UNASSIGNED) {
> -                client->set_memory(client, addr | i << TARGET_PAGE_BITS,
> -                                   TARGET_PAGE_SIZE, pd[i].phys_offset);
> +                target_phys_addr_t start_addr = addr | i << TARGET_PAGE_BITS;
> +
> +                if (map->size &&
> +                    start_addr == map->start_addr + map->size &&
> +                    pd[i].phys_offset == map->phys_offset + map->size) {
> +
> +                    map->size += TARGET_PAGE_SIZE;
> +                    continue;
> +                } else if (map->size) {
> +                    client->set_memory(client, map->start_addr,
> +                                       map->size, map->phys_offset);
> +                }
> +
> +                map->start_addr = start_addr;
> +                map->size = TARGET_PAGE_SIZE;
> +                map->phys_offset = pd[i].phys_offset;
>              }
>          }
>      } else {
>          void **pp = *lp;
>          for (i = 0; i < L2_SIZE; ++i) {
>              phys_page_for_each_1(client, level - 1, pp + i,
> -                                 (addr << L2_BITS) | i);
> +                                 (addr << L2_BITS) | i, map);
>          }
>      }
>  }
> @@ -1776,9 +1797,14 @@ static void phys_page_for_each_1(CPUPhysMemoryClient *client,
>  static void phys_page_for_each(CPUPhysMemoryClient *client)
>  {
>      int i;
> +    struct last_map map = { 0 };
> +

Nit: just {} is enough.

>      for (i = 0; i < P_L1_SIZE; ++i) {
>          phys_page_for_each_1(client, P_L1_SHIFT / L2_BITS - 1,
> -                             l1_phys_map + i, i);
> +                             l1_phys_map + i, i, &map);
> +    }
> +    if (map.size) {
> +        client->set_memory(client, map.start_addr, map.size, map.phys_offset);
>      }
>  }
>