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); + if (!access_ok(VERIFY_WRITE, p, 1)) + return -EFAULT; + + if (__get_user(val, p)) + return -EFAULT; + + val |= 1U << (nr % BITS_PER_BYTE); + if (__put_user(val, p)) + return -EFAULT; + + return 0; +} + /* more complex routines */ -- 1.7.0.4 -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html