[PATCH 5/5] vhost: translate_desc: optimization for desc.len < region size

Igor Mammedov
2015-06-16T16:34:19Z

when translating descriptors they are typically less than
memory region that holds them and translated into 1 iov
enty, so it's not nessesary to check remaining length
twice and calculate used length and next address
in such cases.

so relace a remaining length and 'size' increment branches
with a single remaining length check and execute
next iov steps only when it needed.

It saves tiny 2% of translate_desc() execution time.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
PS:
I'm not sure if iov_size > 0 is always true, if it's not
then better to drop this patch.
---
drivers/vhost/vhost.c | 21 +++++++++++++--------
1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 68c1c88..84c457d 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1111,12 +1111,8 @@ static int translate_desc(struct vhost_virtqueue *vq, u64 addr, u32 len,
int ret = 0;

mem = vq->memory;
- while ((u64)len > s) {
+ do {
u64 size;
- if (unlikely(ret >= iov_size)) {
- ret = -ENOBUFS;
- break;
- }
reg = find_region(mem, addr, len, &vq->cached_reg);
if (unlikely(!reg)) {
ret = -EFAULT;
@@ -1124,13 +1120,22 @@ static int translate_desc(struct vhost_virtqueue *vq, u64 addr, u32 len,
}
_iov = iov + ret;
size = reg->memory_size - addr + reg->guest_phys_addr;
- _iov->iov_len = min((u64)len - s, size);
_iov->iov_base = (void __user *)(unsigned long)
(reg->userspace_addr + addr - reg->guest_phys_addr);
+ ++ret;
+ if (likely((u64)len - s < size)) {
+ _iov->iov_len = (u64)len - s;
+ break;
+ }
+
+ if (unlikely(ret >= iov_size)) {
+ ret = -ENOBUFS;
+ break;
+ }
+ _iov->iov_len = size;
s += size;
addr += size;
- ++ret;
- }
+ } while (1);

return ret;
}
--
1.8.3.1