linux-kernel - Re: [PATCH 5/5] vhost: translate_desc: optimization for desc.len

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Date:	Tue, 16 Jun 2015 23:11:54 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Igor Mammedov <imammedo@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	pbonzini@...hat.com
Subject: Re: [PATCH 5/5] vhost: translate_desc: optimization for desc.len <
 region size

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@...hat.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
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