linux-kernel - Re: [RFC PATCH] iommu/virtio: __viommu_sync

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Message-ID: <YyxSAX9UpAyoq01y@myrica>
Date:   Thu, 22 Sep 2022 13:16:01 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Liu Song <liusong@...ux.alibaba.com>
Cc:     joro@...tes.org, will@...nel.org, robin.murphy@....com,
        virtualization@...ts.linux-foundation.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] iommu/virtio: __viommu_sync_req is no need to return
 a value

Hi Liu,

On Thu, Sep 22, 2022 at 07:24:46PM +0800, Liu Song wrote:
> From: Liu Song <liusong@...ux.alibaba.com>
> 
> In "__viommu_sync_req", 0 is always returned as the only return value, no
> return value is needed for this case, and the processes and functions
> involved are adjusted accordingly.
> 
> Signed-off-by: Liu Song <liusong@...ux.alibaba.com>

Thanks for the patch but I'd rather improve __viommu_sync_req() to handle
more errors. At the moment, if the virtqueue breaks then it spins
infinitely waiting for a host response. We should at least check the
return value of virtqueue_kick(), and maybe add a timeout as well although
I'm not sure which time base we can use reliably here.

Thanks,
Jean

> ---
>  drivers/iommu/virtio-iommu.c | 23 ++++++-----------------
>  1 file changed, 6 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/iommu/virtio-iommu.c b/drivers/iommu/virtio-iommu.c
> index b7c2280..fde5661 100644
> --- a/drivers/iommu/virtio-iommu.c
> +++ b/drivers/iommu/virtio-iommu.c
> @@ -151,7 +151,7 @@ static off_t viommu_get_write_desc_offset(struct viommu_dev *viommu,
>   * Wait for all added requests to complete. When this function returns, all
>   * requests that were in-flight at the time of the call have completed.
>   */
> -static int __viommu_sync_req(struct viommu_dev *viommu)
> +static void __viommu_sync_req(struct viommu_dev *viommu)
>  {
>  	unsigned int len;
>  	size_t write_len;
> @@ -180,22 +180,15 @@ static int __viommu_sync_req(struct viommu_dev *viommu)
>  		list_del(&req->list);
>  		kfree(req);
>  	}
> -
> -	return 0;
>  }
>  
> -static int viommu_sync_req(struct viommu_dev *viommu)
> +static void viommu_sync_req(struct viommu_dev *viommu)
>  {
> -	int ret;
>  	unsigned long flags;
>  
>  	spin_lock_irqsave(&viommu->request_lock, flags);
> -	ret = __viommu_sync_req(viommu);
> -	if (ret)
> -		dev_dbg(viommu->dev, "could not sync requests (%d)\n", ret);
> +	__viommu_sync_req(viommu);
>  	spin_unlock_irqrestore(&viommu->request_lock, flags);
> -
> -	return ret;
>  }
>  
>  /*
> @@ -247,8 +240,8 @@ static int __viommu_add_req(struct viommu_dev *viommu, void *buf, size_t len,
>  	ret = virtqueue_add_sgs(vq, sg, 1, 1, req, GFP_ATOMIC);
>  	if (ret == -ENOSPC) {
>  		/* If the queue is full, sync and retry */
> -		if (!__viommu_sync_req(viommu))
> -			ret = virtqueue_add_sgs(vq, sg, 1, 1, req, GFP_ATOMIC);
> +		__viommu_sync_req(viommu);
> +		ret = virtqueue_add_sgs(vq, sg, 1, 1, req, GFP_ATOMIC);
>  	}
>  	if (ret)
>  		goto err_free;
> @@ -293,11 +286,7 @@ static int viommu_send_req_sync(struct viommu_dev *viommu, void *buf,
>  		goto out_unlock;
>  	}
>  
> -	ret = __viommu_sync_req(viommu);
> -	if (ret) {
> -		dev_dbg(viommu->dev, "could not sync requests (%d)\n", ret);
> -		/* Fall-through (get the actual request status) */
> -	}
> +	__viommu_sync_req(viommu);
>  
>  	ret = viommu_get_req_errno(buf, len);
>  out_unlock:
> -- 
> 1.8.3.1
>