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

[Michael S. Tsirkin]

On Thu, May 05, 2011 at 08:21:06AM -0600, Alex Williamson wrote:
> On Thu, 2011-05-05 at 16:21 +0300, Michael S. Tsirkin wrote:
> > 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 <address@hidden>
> > > ---
> > > 
> > >  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?
> 
> struct CPUPhysMemoryClient {
>     void (*set_memory)(struct CPUPhysMemoryClient *client,
>                        target_phys_addr_t start_addr,
>                        ram_addr_t size,
>                        ram_addr_t phys_offset);
> 
> ram_addr_t seems to be the standard for describing these types of
> things.  It's an unsigned long, so 4G is only  concern for 32b builds,
> which don't support that much memory anyway.  Please apply.  Thanks,
> 
> Alex

OK, I don't think it's a problem in practice.
I dislike the use of _addr for sizes, we should
have _size_t, but that's a separate problem,
this patch is consistent.

I'll give people a bit of time to review and reply though,
there seems to be no rush.

> > > +    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);
> > >      }
> > >  }
> > >  
> 
>