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

Michael S. Tsirkin
2015-06-16T21:12:06Z

On Tue, Jun 16, 2015 at 06:33:39PM +0200, Igor Mammedov wrote:
> when translating descriptors they are typically less than
> memory region that holds them and translated into 1 iov
> enty,

entry

> so it's not nessesary to check remaining length
> twice and calculate used length and next address
> in such cases.
>
> so relace

replace

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

it is needed

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

a tiny

>
> 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