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Subject[RFC][PATCH 7/12 not tested yet] PPC: introduce __set_bit() like function for bitmaps in user space
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 <>
Signed-off-by: Fernando Luis Vazquez Cao <>
CC: Alexander Graf <>
CC: Benjamin Herrenschmidt <>
CC: Paul Mackerras <>
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 */


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