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

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Message-ID: <1396423941.26869.64.camel@dagon.hellion.org.uk>
Date:	Wed, 2 Apr 2014 08:32:21 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Zoltan Kiss <zoltan.kiss@...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 Tue, 2014-04-01 at 20:09 +0100, Zoltan Kiss wrote:
> 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.

OK, that makes sense.

> >>   [...]
> >>   		__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.

It's not only about multiple uses of the pattern but about code clarity
and API consistency.

> >> -	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.

Whether you choose to do it now or later this code should use a
consistent set of ops, either gnttab_batch_* or gnttab_*_refs, from day
one.

Ian.

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