Subject: Re: [RFC][PATCH 7/12 not tested yet] PPC: introduce __set_bit() like function for bitmaps in user space

From: Alexander Graf <[email-masked]>
Date: Tue, 11 May 2010 18:00:42 +0200

Takuya Yoshikawa wrote:
> During the work of KVM's dirty page logging optimization, we encountered
> the need of manipulating bitmaps in user space efficiantly. To achive this,
> we introduce a uaccess function for setting a bit in user space following
> Avi's suggestion.
>
>   KVM is now using dirty bitmaps for live-migration and VGA. Although we need
>   to update them from kernel side, copying them every time for updating the
>   dirty log is a big bottleneck. Especially, we tested that zero-copy bitmap
>   manipulation improves responses of GUI manipulations a lot.
>
> We also found one similar need in drivers/vhost/vhost.c in which the author
> implemented set_bit_to_user() locally using inefficient functions: see TODO
> at the top of that.
>
> Probably, this kind of need would be common for virtualization area.
>
> So we introduce a function set_bit_user_non_atomic().
>
> Signed-off-by: Takuya Yoshikawa <[email-masked]>
> Signed-off-by: Fernando Luis Vazquez Cao <[email-masked]>
> CC: Alexander Graf <[email-masked]>
> CC: Benjamin Herrenschmidt <[email-masked]>
> CC: Paul Mackerras <[email-masked]>
> ---
>  arch/powerpc/include/asm/uaccess.h |   19 +++++++++++++++++++
>  1 files changed, 19 insertions(+), 0 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> index 3a01ce8..f878326 100644
> --- a/arch/powerpc/include/asm/uaccess.h
> +++ b/arch/powerpc/include/asm/uaccess.h
> @@ -321,6 +321,25 @@ do {								\
>  	__gu_err;						\
>  })
>  
> +static inline int set_bit_user_non_atomic(int nr, void __user *addr)
> +{
> +	u8 __user *p;
> +	u8 val;
> +
> +	p = (u8 __user *)((unsigned long)addr + nr / BITS_PER_BYTE);
>   

Does C do the + or the / first? Either way, I'd like to see brackets here :)

Alex