linux-kernel - Re: [PATCH v2 2/4] vhost: Drop variable last_avail_idx in vhost_get_vq

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Date: Tue, 30 Apr 2024 09:00:21 +1000
From: Gavin Shan <gshan@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org,
 jasowang@...hat.com, shan.gavin@...il.com
Subject: Re: [PATCH v2 2/4] vhost: Drop variable last_avail_idx in
 vhost_get_vq_desc()

On 4/30/24 04:45, Michael S. Tsirkin wrote:
> On Mon, Apr 29, 2024 at 08:13:58PM +1000, Gavin Shan wrote:
>> The local variable @last_avail_idx is equivalent to vq->last_avail_idx.
>> So the code can be simplified a bit by dropping the local variable
>> @last_avail_idx.
>>
>> No functional change intended.
>>
>> Signed-off-by: Gavin Shan <gshan@...hat.com>
>> ---
>>   drivers/vhost/vhost.c | 7 +++----
>>   1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 7aa623117aab..b278c0333a66 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -2524,7 +2524,6 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>>   {
>>   	struct vring_desc desc;
>>   	unsigned int i, head, found = 0;
>> -	u16 last_avail_idx = vq->last_avail_idx;
>>   	__virtio16 ring_head;
>>   	int ret, access;
>>   
>> @@ -2539,10 +2538,10 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>>   
>>   	/* Grab the next descriptor number they're advertising, and increment
>>   	 * the index we've seen. */
>> -	if (unlikely(vhost_get_avail_head(vq, &ring_head, last_avail_idx))) {
>> +	if (unlikely(vhost_get_avail_head(vq, &ring_head, vq->last_avail_idx))) {
>>   		vq_err(vq, "Failed to read head: idx %d address %p\n",
>> -		       last_avail_idx,
>> -		       &vq->avail->ring[last_avail_idx % vq->num]);
>> +		       vq->last_avail_idx,
>> +		       &vq->avail->ring[vq->last_avail_idx % vq->num]);
>>   		return -EFAULT;
>>   	}
> 
> I don't see the big advantage and the line is long now.
> 

The point is to avoid the local variable @last_avail_idx since it's equivalent
to vq->last_avail_idx, as stated in the commit log. Besides, it paves the way
for PATCH[v2 3/4] where the whole logic fetching the head and sanity check is
moved to vhost_get_avail_head(), so that vhost_get_vq_desc() is simplified

I will drop PATCH[2, 3, 4] as you suggested.

Thanks,
Gavin