netdev - Re: [PATCH 1/1] drivers/vhost : Removes unnecessary 'else' in vhost_copy_from

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Date:   Wed, 18 Dec 2019 13:53:15 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Leonardo Bras <leonardo@...ux.ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] drivers/vhost : Removes unnecessary 'else' in
 vhost_copy_from_user


On 2019/12/13 上午5:15, Leonardo Bras wrote:
> There is no need for this else statement, given that if block will return.
> This change is not supposed to change the output binary.
>
> It reduces identation level on most lines in this function, and also
> fixes an split string on vq_err().
>
> Signed-off-by: Leonardo Bras <leonardo@...ux.ibm.com>
> ---
>   drivers/vhost/vhost.c | 50 +++++++++++++++++++++----------------------
>   1 file changed, 24 insertions(+), 26 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index f44340b41494..b23d1b74c32f 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -824,34 +824,32 @@ static int vhost_copy_from_user(struct vhost_virtqueue *vq, void *to,
>   
>   	if (!vq->iotlb)
>   		return __copy_from_user(to, from, size);
> -	else {
> -		/* This function should be called after iotlb
> -		 * prefetch, which means we're sure that vq
> -		 * could be access through iotlb. So -EAGAIN should
> -		 * not happen in this case.
> -		 */
> -		void __user *uaddr = vhost_vq_meta_fetch(vq,
> -				     (u64)(uintptr_t)from, size,
> -				     VHOST_ADDR_DESC);
> -		struct iov_iter f;
>   
> -		if (uaddr)
> -			return __copy_from_user(to, uaddr, size);
> +	/* This function should be called after iotlb
> +	 * prefetch, which means we're sure that vq
> +	 * could be access through iotlb. So -EAGAIN should
> +	 * not happen in this case.
> +	 */
> +	void __user *uaddr = vhost_vq_meta_fetch(vq,
> +			     (u64)(uintptr_t)from, size,
> +			     VHOST_ADDR_DESC);
> +	struct iov_iter f;


I think this will lead at least warnings from compiler.

Generally, I would not bother to make changes like this especially 
consider it will bring troubles when trying to backporting fixes to 
downstream in the future.

There're some more interesting things: e.g current metadata IOTLB 
performance is bad for dynamic mapping since it will be reset each time 
a new updating is coming.

We can optimize this by only reset the metadata IOTLB when the updating 
is for metdata.

Want to try this?

Thanks


>   
> -		ret = translate_desc(vq, (u64)(uintptr_t)from, size, vq->iotlb_iov,
> -				     ARRAY_SIZE(vq->iotlb_iov),
> -				     VHOST_ACCESS_RO);
> -		if (ret < 0) {
> -			vq_err(vq, "IOTLB translation failure: uaddr "
> -			       "%p size 0x%llx\n", from,
> -			       (unsigned long long) size);
> -			goto out;
> -		}
> -		iov_iter_init(&f, READ, vq->iotlb_iov, ret, size);
> -		ret = copy_from_iter(to, size, &f);
> -		if (ret == size)
> -			ret = 0;
> -	}
> +	if (uaddr)
> +		return __copy_from_user(to, uaddr, size);
> +
> +	ret = translate_desc(vq, (u64)(uintptr_t)from, size, vq->iotlb_iov,
> +			     ARRAY_SIZE(vq->iotlb_iov),
> +			     VHOST_ACCESS_RO);
> +	if (ret < 0) {
> +		vq_err(vq, "IOTLB translation failure: uaddr %p size 0x%llx\n",
> +		       from, (unsigned long long)size);
> +		goto out;
> +	}
> +	iov_iter_init(&f, READ, vq->iotlb_iov, ret, size);
> +	ret = copy_from_iter(to, size, &f);
> +	if (ret == size)
> +		ret = 0;
>   
>   out:
>   	return ret;