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

From: David Vrabel <[email-masked]>
Date: Wed, 2 Apr 2014 14:11:59 +0100

On 01/04/14 12:40, Ian Campbell wrote:
> On Mon, 2014-03-31 at 16:08 +0100, Zoltan Kiss wrote:
>>  
>>  		__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?

A set of 3 might be useful I think.

gnttab_set_copy_op_ref_to_gfn()
gnttab_set_copy_op_gfn_to_ref()
gnttab_set_copy_op_ref_to_ref()

> 
>> -	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).

gnttab_batch_map() is not correct here since it does not update the p2m.
 There is only one copy API (gnttab_batch_copy()).

> 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.

Both mapping and copy need to handle (hypervisor) paging/shaing and the
two calls Zoli has used do this.

gnttab_map_refs() is basically gnttab_batch_map() +
set_foreign_p2m_mapping().

I'd be in favour of a patch that:

- renamed gnttab_map_refs() to gnttab_batch_map_pages()
- refactored it to call gnttab_batch_map().
- added documentation

But I don't see why this would be a prerequisite for this series.

David