Re: [Qemu-devel] [PATCH 14/23] memory: transaction API

[Anthony Liguori <anthony@codemonkey.ws>]

On 07/25/2011 09:02 AM, 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>

What's the motivation for this?  Was this just because it seemed neat to 
do or did you run into a performance issue you were trying to solve?


> ---
>   memory.c |   20 ++++++++++++++++++++
>   memory.h |    8 ++++++++
>   2 files changed, 28 insertions(+), 0 deletions(-)
>
> diff --git a/memory.c b/memory.c
> index 009ad33..370e189 100644
> --- a/memory.c
> +++ b/memory.c
> @@ -18,6 +18,8 @@
>   #include "kvm.h"
>   #include<assert.h>
>
> +unsigned memory_region_transaction_depth = 0;

Shouldn't this be static?

Regards,

Anthony Liguori

> +
>   typedef struct AddrRange AddrRange;
>
>   struct AddrRange {
> @@ -623,6 +625,10 @@ 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);
>       }
> @@ -631,6 +637,20 @@ static void memory_region_update_topology(void)
>       }
>   }
>
> +void memory_region_transaction_begin(void)
> +{
> +    ++memory_region_transaction_depth;
> +}
> +
> +void memory_region_transaction_commit(void)
> +{
> +    if (!memory_region_transaction_depth) {
> +        abort();
> +    }
> +    --memory_region_transaction_depth;
> +    memory_region_update_topology();
> +}
> +
>   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