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 macro set_bit_user_non_atomic() following the implementation
style of x86's uaccess functions.
Note: there is a one restriction to this macro: bitmaps must be 64-bit
aligned (see the comment in this patch).
Signed-off-by: Takuya Yoshikawa <yoshikawa.tak...@oss.ntt.co.jp>
Signed-off-by: Fernando Luis Vazquez Cao <ferna...@oss.ntt.co.jp>
CC: Avi Kivity <a...@redhat.com>
Cc: Thomas Gleixner <t...@linutronix.de>
CC: Ingo Molnar <mi...@redhat.com>
Cc: "H. Peter Anvin" <h...@zytor.com>
---
arch/x86/include/asm/uaccess.h | 39 +++++++++++++++++++++++++++++++++++++++
1 files changed, 39 insertions(+), 0 deletions(-)
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index abd3e0e..3138e65 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -98,6 +98,45 @@ struct exception_table_entry {
extern int fixup_exception(struct pt_regs *regs);
+/**
+ * set_bit_user_non_atomic: - set a bit of a bitmap in user space.
+ * @nr: Bit offset.
+ * @addr: Base address of a bitmap in user space.
+ *
+ * Context: User context only. This function may sleep.
+ *
+ * This macro set a bit of a bitmap in user space.
+ *
+ * Restriction: the bitmap pointed to by @addr must be 64-bit aligned:
+ * the kernel accesses the bitmap by its own word length, so bitmaps
+ * allocated by 32-bit processes may cause fault.
+ *
+ * Returns zero on success, or -EFAULT on error.
+ */
+#define __set_bit_user_non_atomic_asm(nr, addr, err, errret) \
+ asm volatile("1: bts %1,%2\n" \
+ "2:\n" \
+ ".section .fixup,\"ax\"\n" \
+ "3: mov %3,%0\n" \
+ " jmp 2b\n" \
+ ".previous\n" \
+ _ASM_EXTABLE(1b, 3b) \
+ : "=r"(err) \
+ : "r" (nr), "m" (__m(addr)), "i" (errret), "0" (err))
+
+#define set_bit_user_non_atomic(nr, addr) \
+({ \
+ int __ret_sbu; \
+ \
+ might_fault(); \
+ if (access_ok(VERIFY_WRITE, addr, nr/8 + 1)) \
+ __set_bit_user_non_atomic_asm(nr, addr, __ret_sbu, -EFAULT);\
+ else \
+ __ret_sbu = -EFAULT; \
+ \
+ __ret_sbu; \
+})
+
/*
* These are the main single-value transfer routines. They automatically
* use the right size if we just have the right pointer type.
--
1.7.0.4
_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@lists.ozlabs.org
https://lists.ozlabs.org/listinfo/linuxppc-dev
