Re: [PATCH] memory: avoid unnecessary iteration when updating ioeventfds

[David Hildenbrand <david@redhat.com>]

On 28.02.23 15:25, Longpeng(Mike) via wrote:
> From: Longpeng <longpeng2@huawei.com>
> 
> When updating ioeventfds, we need to iterate all address spaces and
> iterate all flat ranges of each address space. There is so much
> redundant process that a FlatView would be iterated for so many times
> during one commit (memory_region_transaction_commit).
> 
> We can mark a FlatView as UPDATED and then skip it in the next iteration
> and clear the UPDATED flag at the end of the commit. The overhead can
> be significantly reduced.
> 
> For example, a VM with 16 vdpa net devices and each one has 65 vectors,
> can reduce the time spent on memory_region_transaction_commit by 95%.
> 
> Signed-off-by: Longpeng <longpeng2@huawei.com>
> ---
>   include/exec/memory.h |  2 ++
>   softmmu/memory.c      | 28 +++++++++++++++++++++++++++-
>   2 files changed, 29 insertions(+), 1 deletion(-)
> 
> diff --git a/include/exec/memory.h b/include/exec/memory.h
> index 2e602a2fad..974eabf765 100644
> --- a/include/exec/memory.h
> +++ b/include/exec/memory.h
> @@ -1093,6 +1093,8 @@ struct FlatView {
>       unsigned nr_allocated;
>       struct AddressSpaceDispatch *dispatch;
>       MemoryRegion *root;
> +#define FLATVIEW_FLAG_IOEVENTFD_UPDATED (1 << 0)
> +    unsigned flags;
>   };
>   
>   static inline FlatView *address_space_to_flatview(AddressSpace *as)
> diff --git a/softmmu/memory.c b/softmmu/memory.c
> index 9d64efca26..71ff996712 100644
> --- a/softmmu/memory.c
> +++ b/softmmu/memory.c
> @@ -815,6 +815,15 @@ FlatView *address_space_get_flatview(AddressSpace *as)
>       return view;
>   }
>   
> +static void address_space_reset_view_flags(AddressSpace *as, unsigned mask)
> +{
> +    FlatView *view = address_space_get_flatview(as);
> +
> +    if (view->flags & mask) {
> +        view->flags &= ~mask;
> +    }

Just do it unconditionally.

Unfortunately, I cannot comment on the bigger picture, but it does look 
good to me.

-- 
Thanks,

David / dhildenb