netdev - Re: [PATCH net-next v2 2/2] xen-netback: Grant copy the header instead of map and memcpy

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Date:	Tue, 1 Apr 2014 20:09:44 +0100
From:	Zoltan Kiss <zoltan.kiss@...rix.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	<wei.liu2@...rix.com>, <xen-devel@...ts.xenproject.org>,
	<paul.durrant@...rix.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <jonathan.davies@...rix.com>
Subject: Re: [PATCH net-next v2 2/2] xen-netback: Grant copy the header instead
 of map and memcpy

On 01/04/14 12:40, Ian Campbell wrote:
> On Mon, 2014-03-31 at 16:08 +0100, Zoltan Kiss wrote:
>>
>>   check_frags:
>> -	for (i = start; i < nr_frags; i++) {
>> +	for (i = 0; i < nr_frags; i++, gop_map++) {
>>   		int j, newerr;
>>
>>   		pending_idx = frag_get_pending_idx(&shinfo->frags[i]);
>> -		tx_info = &vif->pending_tx_info[pending_idx];
>>
>>   		/* Check error status: if okay then remember grant handle. */
>> -		newerr = (++gop_map)->status;
>> +		newerr = (gop_map)->status;
>
> You've reworked the handling of gop_map and when and where it is
> incremented, which might be a legit cleanup but does it relate to the
> bulk of this change somehow that I'm missing?
That original "++gop_map" assumed the header was also grant mapped, and 
incremented the pointer first here, which is wrong now.

>
>>   [...]
>>   		__skb_put(skb, data_len);
>> +		vif->tx_copy_ops[*copy_ops].source.u.ref = txreq.gref;
>> +		vif->tx_copy_ops[*copy_ops].source.domid = vif->domid;
>> +		vif->tx_copy_ops[*copy_ops].source.offset = txreq.offset;
>> +
>> +		vif->tx_copy_ops[*copy_ops].dest.u.gmfn =
>> +			virt_to_mfn(skb->data);
>> +		vif->tx_copy_ops[*copy_ops].dest.domid = DOMID_SELF;
>> +		vif->tx_copy_ops[*copy_ops].dest.offset =
>> +			offset_in_page(skb->data);
>> +
>> +		vif->tx_copy_ops[*copy_ops].len = data_len;
>> +		vif->tx_copy_ops[*copy_ops].flags = GNTCOPY_source_gref;
>
> We have gnttab_set_map_op. Should we have gnttap_set_copy_op too?
This is the only place at the moment when we do this, so I wouldn't 
bother to do it.

>
>> -	BUG_ON(ret);
>> +	else {
>> +		gnttab_batch_copy(vif->tx_copy_ops, nr_cops);
>> +		if (nr_mops != 0) {
>
>
> if (nr_mops) would do.
>
>> +			ret = gnttab_map_refs(vif->tx_map_ops,
>
> So we use gnttab_batch_copy and gnttab_map_refs.
>
> Shouldn't we either use gnttab_batch_copy and gnttab_batch_map or
> gnttab_copy gnttab_map_refs. (where gnttab_copy might be a bare
> GNTTABOP_copy or might be a helper wrapper).
>
> The point of the batch interface is to handle page unsharing etc, but
> doing it only for copies seems like a waste one way or another.
The difference between gnttab_batch_map and gnttab_map_refs is that the 
latter calls set_foreign_p2m_mapping, which we need for sure.
gnttab_batch_copy calls the hypercall and tries again if op->status == 
GNTST_eagain. I think that's exactly what we need here as well.
The naming might be confusing indeed, but that should be the topic of an 
another patch.

Zoli
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