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From: Takuya Yoshikawa on 4 May 2010 09:10
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(a)oss.ntt.co.jp>
Signed-off-by: Fernando Luis Vazquez Cao <fernando(a)oss.ntt.co.jp>
CC: Alexander Graf <agraf(a)suse.de>
CC: Benjamin Herrenschmidt <benh(a)kernel.crashing.org>
CC: Paul Mackerras <paulus(a)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

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Prev: KEYS: Use RCU dereference wrappers in keyring key type code
Next: [RFC][PATCH 4/12] x86: introduce copy_in_user() for 32-bit