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[Qemu-devel] [PULL 1/3] iov: avoid memcpy for "simple" iov_from_buf/iov_
From: |
Stefan Hajnoczi |
Subject: |
[Qemu-devel] [PULL 1/3] iov: avoid memcpy for "simple" iov_from_buf/iov_to_buf |
Date: |
Tue, 9 Feb 2016 15:11:29 +0000 |
From: Paolo Bonzini <address@hidden>
memcpy can take a large amount of time for small reads and writes.
For virtio it is a common case that the first iovec can satisfy the
whole read or write. In that case, and if bytes is a constant to
avoid excessive growth of code, inline the first iteration
into the caller.
Signed-off-by: Paolo Bonzini <address@hidden>
Message-id: address@hidden
Signed-off-by: Stefan Hajnoczi <address@hidden>
---
include/qemu/iov.h | 34 ++++++++++++++++++++++++++++++----
util/iov.c | 8 ++++----
2 files changed, 34 insertions(+), 8 deletions(-)
diff --git a/include/qemu/iov.h b/include/qemu/iov.h
index 569b2c2..2847551 100644
--- a/include/qemu/iov.h
+++ b/include/qemu/iov.h
@@ -39,10 +39,36 @@ size_t iov_size(const struct iovec *iov, const unsigned int
iov_cnt);
* such "large" value is -1 (sinice size_t is unsigned),
* so specifying `-1' as `bytes' means 'up to the end of iovec'.
*/
-size_t iov_from_buf(const struct iovec *iov, unsigned int iov_cnt,
- size_t offset, const void *buf, size_t bytes);
-size_t iov_to_buf(const struct iovec *iov, const unsigned int iov_cnt,
- size_t offset, void *buf, size_t bytes);
+size_t iov_from_buf_full(const struct iovec *iov, unsigned int iov_cnt,
+ size_t offset, const void *buf, size_t bytes);
+size_t iov_to_buf_full(const struct iovec *iov, const unsigned int iov_cnt,
+ size_t offset, void *buf, size_t bytes);
+
+static inline size_t
+iov_from_buf(const struct iovec *iov, unsigned int iov_cnt,
+ size_t offset, const void *buf, size_t bytes)
+{
+ if (__builtin_constant_p(bytes) && iov_cnt &&
+ offset <= iov[0].iov_len && bytes <= iov[0].iov_len - offset) {
+ memcpy(iov[0].iov_base + offset, buf, bytes);
+ return bytes;
+ } else {
+ return iov_from_buf_full(iov, iov_cnt, offset, buf, bytes);
+ }
+}
+
+static inline size_t
+iov_to_buf(const struct iovec *iov, const unsigned int iov_cnt,
+ size_t offset, void *buf, size_t bytes)
+{
+ if (__builtin_constant_p(bytes) && iov_cnt &&
+ offset <= iov[0].iov_len && bytes <= iov[0].iov_len - offset) {
+ memcpy(buf, iov[0].iov_base + offset, bytes);
+ return bytes;
+ } else {
+ return iov_to_buf_full(iov, iov_cnt, offset, buf, bytes);
+ }
+}
/**
* Set data bytes pointed out by iovec `iov' of size `iov_cnt' elements,
diff --git a/util/iov.c b/util/iov.c
index e802ee1..062f4e5 100644
--- a/util/iov.c
+++ b/util/iov.c
@@ -20,8 +20,8 @@
#include "qemu/iov.h"
#include "qemu/sockets.h"
-size_t iov_from_buf(const struct iovec *iov, unsigned int iov_cnt,
- size_t offset, const void *buf, size_t bytes)
+size_t iov_from_buf_full(const struct iovec *iov, unsigned int iov_cnt,
+ size_t offset, const void *buf, size_t bytes)
{
size_t done;
unsigned int i;
@@ -39,8 +39,8 @@ size_t iov_from_buf(const struct iovec *iov, unsigned int
iov_cnt,
return done;
}
-size_t iov_to_buf(const struct iovec *iov, const unsigned int iov_cnt,
- size_t offset, void *buf, size_t bytes)
+size_t iov_to_buf_full(const struct iovec *iov, const unsigned int iov_cnt,
+ size_t offset, void *buf, size_t bytes)
{
size_t done;
unsigned int i;
--
2.5.0