Re: [PATCH] memory: transaction API

[Ferry Huberts <mailings@hupie.com>]

On 07/21/2011 12:21 PM, Avi Kivity wrote:
> Allow changes to the memory hierarchy to be accumulated and
> made visible all at once.  This reduces computational effort,
> especially when an accelerator (e.g. kvm) is involved.
> 
> Useful when a single register update causes multiple changes
> to an address space.
> 
> Signed-off-by: Avi Kivity <avi@redhat.com>
> ---
>  memory.c |   20 ++++++++++++++++++++
>  memory.h |    8 ++++++++
>  2 files changed, 28 insertions(+), 0 deletions(-)
> 
> diff --git a/memory.c b/memory.c
> index 0ffc905..8780533 100644
> --- a/memory.c
> +++ b/memory.c
> @@ -18,6 +18,8 @@
>  #include "kvm.h"
>  #include <assert.h>
>  
> +unsigned memory_region_transaction_depth = 0;
> +
>  typedef struct AddrRange AddrRange;
>  
>  struct AddrRange {
> @@ -648,6 +650,10 @@ static void address_space_update_topology(AddressSpace *as)
>  
>  static void memory_region_update_topology(void)
>  {
> +    if (memory_region_transaction_depth) {
> +        return;
> +    }
> +
>      if (address_space_memory.root) {
>          address_space_update_topology(&address_space_memory);
>      }
> @@ -656,6 +662,20 @@ static void memory_region_update_topology(void)
>      }
>  }
>  
> +void memory_region_transaction_begin(void)
> +{
> +    ++memory_region_transaction_depth;
> +}
> +

wouldn't you rather keep it safe by doing either here

if (!memory_region_transaction_depth)
  memory_region_transaction_depth++;

> +void memory_region_transaction_commit(void)
> +{
> +    if (!memory_region_transaction_depth) {
> +        abort();
> +    }


> +    --memory_region_transaction_depth;
> +    memory_region_update_topology();
> +}
> +

or by doing here

while (!memory_region_transaction_depth)
  memory_region_transaction_depth--;


with your setup nesting transactions will not work correctly I think.
You seem to have designed it to not do nesting, so it's safer to make
that explicit in your code imho

therefore I'd go for the change in _begin

also, wouldn't you rather rename memory_region_transaction_depth to
memory_region_transaction_pending?

>  void memory_region_init(MemoryRegion *mr,
>                          const char *name,
>                          uint64_t size)
> diff --git a/memory.h b/memory.h
> index e4c0ad1..cb3a9b6 100644
> --- a/memory.h
> +++ b/memory.h
> @@ -246,6 +246,14 @@ void memory_region_add_subregion_overlap(MemoryRegion *mr,
>  void memory_region_del_subregion(MemoryRegion *mr,
>                                   MemoryRegion *subregion);
>  
> +/* Start a transaction; changes will be accumulated and made visible only
> + * when the transaction ends.
> + */
> +void memory_region_transaction_begin(void);
> +/* Commit a transaction and make changes visible to the guest.
> + */
> +void memory_region_transaction_commit(void);
> +
>  #endif
>  
>  #endif


-- 
Ferry Huberts