netdev - Re: [PATCH net-next 1/3] vhost: generalize adding used elem

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Date:   Fri, 14 Dec 2018 12:00:02 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "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 net-next 1/3] vhost: generalize adding used elem


On 2018/12/14 上午3:41, Michael S. Tsirkin wrote:
> On Thu, Dec 13, 2018 at 06:10:20PM +0800, Jason Wang wrote:
>> Use one generic vhost_copy_to_user() instead of two dedicated
>> accessor. This will simplify the conversion to fine grain accessors.
>>
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>
> The reason we did it like this is because it was faster.
> Want to try benchmarking before we change it?


Faster is a good side effect of such conversion. Otherwise we need two 
new fine grain memory accessor.

But, anyway, I can test its impact.

Thanks


>
>> ---
>>   drivers/vhost/vhost.c | 11 +----------
>>   1 file changed, 1 insertion(+), 10 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 3a5f81a66d34..1c54ec1b82f8 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -2164,16 +2164,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
>>   
>>   	start = vq->last_used_idx & (vq->num - 1);
>>   	used = vq->used->ring + start;
>> -	if (count == 1) {
>> -		if (vhost_put_user(vq, heads[0].id, &used->id)) {
>> -			vq_err(vq, "Failed to write used id");
>> -			return -EFAULT;
>> -		}
>> -		if (vhost_put_user(vq, heads[0].len, &used->len)) {
>> -			vq_err(vq, "Failed to write used len");
>> -			return -EFAULT;
>> -		}
>> -	} else if (vhost_copy_to_user(vq, used, heads, count * sizeof *used)) {
>> +	if (vhost_copy_to_user(vq, used, heads, count * sizeof *used)) {
>>   		vq_err(vq, "Failed to write used");
>>   		return -EFAULT;
>>   	}
>> -- 
>> 2.17.1