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

[Takuya Yoshikawa <takuya.yoshikawa@gmail.com>]

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@oss.ntt.co.jp>
Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
CC: Alexander Graf <agraf@suse.de>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Paul Mackerras <paulus@samba.org>
---
 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