netdev - Re: [RFC PATCH 04/13] xen-netback: implement RX persistent grants

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Message-ID: <20150602150704.GQ19403@zion.uk.xensource.com>
Date:	Tue, 2 Jun 2015 16:07:04 +0100
From:	Wei Liu <wei.liu2@...rix.com>
To:	Joao Martins <Joao.Martins@...lab.eu>
CC:	Wei Liu <wei.liu2@...rix.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"ian.campbell@...rix.com" <ian.campbell@...rix.com>,
	"david.vrabel@...rix.com" <david.vrabel@...rix.com>,
	"boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>
Subject: Re: [RFC PATCH 04/13] xen-netback: implement RX persistent grants

On Fri, May 22, 2015 at 10:25:10AM +0000, Joao Martins wrote:
> 
> On 19 May 2015, at 17:32, Wei Liu <wei.liu2@...rix.com> wrote:
> 
> > On Tue, May 12, 2015 at 07:18:28PM +0200, Joao Martins wrote:
> >> It starts by doing a lookup in the tree for a gref. If no persistent
> >> grant is found on the tree, it will do grant copy and prepare
> >> the grant maps. Finally valides the grant map and adds it to the tree.
> > 
> > validates?
> > 
> >> After mapped these grants can be pulled from the tree in the subsequent
> >> requests. If it's out of pages in the tree pool, it will fallback to
> >> grant copy.
> >> 
> > 
> > Again, this looks complicated. Why use combined scheme? I will do
> > detailed reviews after we're sure we need such scheme.
> When we don't have the gref in tree we need to map it and then copying
> afterwards into the newly mapped page (and this only happens once until
> the grant is in tree). My options here were to either do this explicitly,
> after we add the persistent grant in which we would need to save to
> dst/src address and len to copy. The other option is to reuse the grant
> copy (since it's only once until the grant is in the tree) and use memcpy
> in followings requests. Additionally I allow the fallback to grant copy in

Which approach were you using here? I looked at the code but couldn't
quite get which one you were getting at. I guess the first one?

> case the guest provides providing more grefs > max_grants.
> 
> Note that this is also the case for TX as well, with regard to grant
> copying the header. I was unsure about which one is the most correct way
> of doing it, but ultimately the latter involved a smaller codepath, and
> that's why I chose it. What do you think?
> 

Shorter is better. Easier to understand.

Wei.
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