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 <yoshikawa.takuya@xxxxxxxxxxxxx>
> Signed-off-by: Fernando Luis Vazquez Cao <fernando@xxxxxxxxxxxxx>
> CC: Alexander Graf <agraf@xxxxxxx>
> CC: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> CC: Paul Mackerras <paulus@xxxxxxxxx>
> ---
> 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
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