[PATCH v2 4/6] vhost: translate_desc: optimization for desc.len < region size

[Igor Mammedov <imammedo@redhat.com>]

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

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

It saves a 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 5c39a1e..5bcb323 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