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iov: avoid memcpy for "simple" iov_from_buf/iov_to_buf 

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 <pbonzini@redhat.com>
Message-id: 1450782213-14227-1-git-send-email-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Paolo Bonzini 2015-12-22 12:03:33 +01:00 committed by Stefan Hajnoczi
parent ee8e8f92a7
commit ad523bca56
2 changed files with 34 additions and 8 deletions
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34
include/qemu/iov.h
View File

@ -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,

8
util/iov.c
View File

@ -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;