netdev - Re: [Xen-devel] [RFC PATCH 12/13] xen-netfront: implement TX persistent grants

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Date:	Tue, 19 May 2015 11:23:35 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Joao Martins <Joao.Martins@...lab.eu>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"wei.liu2@...rix.com" <wei.liu2@...rix.com>,
	"ian.campbell@...rix.com" <ian.campbell@...rix.com>,
	"boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] [RFC PATCH 12/13] xen-netfront: implement TX persistent
 grants

On 19/05/15 11:20, Joao Martins wrote:
> 
> On 18 May 2015, at 17:55, David Vrabel <david.vrabel@...rix.com> wrote:
>> On 12/05/15 18:18, Joao Martins wrote:
>>> Instead of grant/revoking the buffer related to the skb, it will use
>>> an already granted page and memcpy  to it. The grants will be mapped
>>> by xen-netback and reused overtime, but only unmapped when the vif
>>> disconnects, as opposed to every packet.
>>>
>>> This only happens if the backend supports persistent grants since it
>>> would, otherwise, introduce the overhead of a memcpy on top of the
>>> grant map.
>> [...]
>>> --- a/drivers/net/xen-netfront.c
>>> +++ b/drivers/net/xen-netfront.c
>> [...]
>>> @@ -1610,7 +1622,10 @@ static int xennet_init_queue(struct netfront_queue *queue)
>>> 	for (i = 0; i < NET_TX_RING_SIZE; i++) {
>>> 		skb_entry_set_link(&queue->tx_skbs[i], i+1);
>>> 		queue->grant_tx[i].ref = GRANT_INVALID_REF;
>>> -		queue->grant_tx[i].page = NULL;
>>> +		if (queue->info->feature_persistent)
>>> +			queue->grant_tx[i].page = alloc_page(GFP_NOIO);
>>
>> Need to check for alloc failure here and unwind correctly?
> Sorry, I overlooked this check. I will fix that.
> 
>> Why NOIO?
> May be I am misusing NOIO where I meant __GFP_WAIT.
> Tough given we are under rtnl_lock() perhaps GFP_ATOMIC should be used instead.

rtnl_lock() is a mutex, so sleeping is allowed, so GFP_KERNEL is fine
here I think.

David
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