Date: Tue, 30 Apr 2024 09:18:53 +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 1/4] vhost: Improve vhost_get_avail_idx() with
 smp_rmb()

On 4/30/24 04:44, Michael S. Tsirkin wrote:
> On Mon, Apr 29, 2024 at 08:13:57PM +1000, Gavin Shan wrote:
>> From: "Michael S. Tsirkin" <mst@...hat.com>
>>
>> All the callers of vhost_get_avail_idx() are concerned to the memory
> 
> *with* the memory barrier
> 

Thanks, will be corrected in v3.

>> barrier, imposed by smp_rmb() to ensure the order of the available
>> ring entry read and avail_idx read.
>>
>> Improve vhost_get_avail_idx() so that smp_rmb() is executed when
>> the avail_idx is advanced.
> 
> accessed, not advanced. guest advances it.
> 

smp_rmb() is executed only when vp->last_avail_idx != vp->avail_idx.
I used 'advanced' to indicate the condition. 'accessed' is also
correct since the 'advanced' case included to 'accessed' case.

>> With it, the callers needn't to worry
>> about the memory barrier.
>>
>> No functional change intended.
> 
> I'd add:
> 
> As a side benefit, we also validate the index on all paths now, which
> will hopefully help catch future errors earlier.
> 
> Note: current code is inconsistent in how it handles errors:
> some places treat it as an empty ring, others - non empty.
> This patch does not attempt to change the existing behaviour.
> 

Ok, I will integrate this to v3's commit log.

> 
> 
>> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>> [gshan: repainted vhost_get_avail_idx()]
> 
> ?repainted?
> 

It's just a indicator to say the changes aren't simply copied from
[1]. Some follow-up changes are also applied. So it needs to be
reviewed. I will drop this in v3.

>> Reviewed-by: Gavin Shan <gshan@...hat.com>
>> Acked-by: Will Deacon <will@...nel.org>
>> ---
>>   drivers/vhost/vhost.c | 106 +++++++++++++++++-------------------------
>>   1 file changed, 42 insertions(+), 64 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 8995730ce0bf..7aa623117aab 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -1290,10 +1290,36 @@ static void vhost_dev_unlock_vqs(struct vhost_dev *d)
>>   		mutex_unlock(&d->vqs[i]->mutex);
>>   }
>>   
>> -static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq,
>> -				      __virtio16 *idx)
>> +static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq)
>>   {
>> -	return vhost_get_avail(vq, *idx, &vq->avail->idx);
>> +	__virtio16 idx;
>> +	int r;
>> +
>> +	r = vhost_get_avail(vq, idx, &vq->avail->idx);
>> +	if (unlikely(r < 0)) {
>> +		vq_err(vq, "Failed to access available index at %p (%d)\n",
>> +		       &vq->avail->idx, r);
>> +		return r;
>> +	}
>> +
>> +	/* Check it isn't doing very strange thing with available indexes */
>> +	vq->avail_idx = vhost16_to_cpu(vq, idx);
>> +	if (unlikely((u16)(vq->avail_idx - vq->last_avail_idx) > vq->num)) {
>> +		vq_err(vq, "Invalid available index change from %u to %u",
>> +		       vq->last_avail_idx, vq->avail_idx);
>> +		return -EINVAL;
>> +	}
>> +
>> +	/* We're done if there is nothing new */
>> +	if (vq->avail_idx == vq->last_avail_idx)
>> +		return 0;
>> +
>> +	/*
>> +	 * We updated vq->avail_idx so we need a memory barrier between
>> +	 * the index read above and the caller reading avail ring entries.
>> +	 */
>> +	smp_rmb();
>> +	return 1;
>>   }
>>   
>>   static inline int vhost_get_avail_head(struct vhost_virtqueue *vq,
>> @@ -2498,38 +2524,17 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>>   {
>>   	struct vring_desc desc;
>>   	unsigned int i, head, found = 0;
>> -	u16 last_avail_idx;
>> -	__virtio16 avail_idx;
>> +	u16 last_avail_idx = vq->last_avail_idx;
>>   	__virtio16 ring_head;
>>   	int ret, access;
>>   
>> -	/* Check it isn't doing very strange things with descriptor numbers. */
>> -	last_avail_idx = vq->last_avail_idx;
>> -
>>   	if (vq->avail_idx == vq->last_avail_idx) {
>> -		if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
>> -			vq_err(vq, "Failed to access avail idx at %p\n",
>> -				&vq->avail->idx);
>> -			return -EFAULT;
>> -		}
>> -		vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>> -
>> -		if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) {
>> -			vq_err(vq, "Guest moved avail index from %u to %u",
>> -				last_avail_idx, vq->avail_idx);
>> -			return -EFAULT;
>> -		}
>> +		ret = vhost_get_avail_idx(vq);
>> +		if (unlikely(ret < 0))
>> +			return ret;
>>   
>> -		/* If there's nothing new since last we looked, return
>> -		 * invalid.
>> -		 */
>> -		if (vq->avail_idx == last_avail_idx)
>> +		if (!ret)
>>   			return vq->num;
>> -
>> -		/* Only get avail ring entries after they have been
>> -		 * exposed by guest.
>> -		 */
>> -		smp_rmb();
>>   	}
>>   
>>   	/* Grab the next descriptor number they're advertising, and increment
>> @@ -2790,35 +2795,20 @@ EXPORT_SYMBOL_GPL(vhost_add_used_and_signal_n);
>>   /* return true if we're sure that avaiable ring is empty */
>>   bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>>   {
>> -	__virtio16 avail_idx;
>>   	int r;
>>   
>>   	if (vq->avail_idx != vq->last_avail_idx)
>>   		return false;
>>   
>> -	r = vhost_get_avail_idx(vq, &avail_idx);
>> -	if (unlikely(r))
>> -		return false;
>> -
>> -	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>> -	if (vq->avail_idx != vq->last_avail_idx) {
>> -		/* Since we have updated avail_idx, the following
>> -		 * call to vhost_get_vq_desc() will read available
>> -		 * ring entries. Make sure that read happens after
>> -		 * the avail_idx read.
>> -		 */
>> -		smp_rmb();
>> -		return false;
>> -	}
>> -
>> -	return true;
>> +	/* Treat error as non-empty here */
> 
> If you write the comment like that then put it before "return":
> that is where you treat an error like this.
> And I feel Note: is better in that the comment does not
> explain all of what is going on, just an aspect of it.
> 

Ok. Will do in v3.

>> +	r = vhost_get_avail_idx(vq);
>> +	return r == 0;
>>   }
>>   EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
>>   
>>   /* OK, now we need to know about added descriptors. */
>>   bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>>   {
>> -	__virtio16 avail_idx;
>>   	int r;
>>   
>>   	if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
>> @@ -2842,25 +2832,13 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>>   	/* They could have slipped one in as we were doing that: make
>>   	 * sure it's written, then check again. */
>>   	smp_mb();
>> -	r = vhost_get_avail_idx(vq, &avail_idx);
>> -	if (r) {
>> -		vq_err(vq, "Failed to check avail idx at %p: %d\n",
>> -		       &vq->avail->idx, r);
>> -		return false;
>> -	}
>>   
>> -	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>> -	if (vq->avail_idx != vq->last_avail_idx) {
>> -		/* Since we have updated avail_idx, the following
>> -		 * call to vhost_get_vq_desc() will read available
>> -		 * ring entries. Make sure that read happens after
>> -		 * the avail_idx read.
>> -		 */
>> -		smp_rmb();
>> -		return true;
>> -	}
>> +	/* Treat error as empty here */
>> +	r = vhost_get_avail_idx(vq);
> 
> If you write the comment like that then put it before "return":
> that is where you treat an error like this.
> And I feel Note: is better in that the comment does not
> explain all of what is going on, just an aspect of it.
> 

Sure, will do in v3.

>> +	if (unlikely(r < 0))
>> +		return false;
>>   
>> -	return false;
>> +	return r;
>>   }
>>   EXPORT_SYMBOL_GPL(vhost_enable_notify);


Thanks,
Gavin

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