linux-kernel - Re: [RFC PATCH 2/2] macvtap: TX zero copy between guest and host kernel

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Message-ID: <20100915053059.GB25481@redhat.com>
Date:	Wed, 15 Sep 2010 07:31:00 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	Arnd Bergmann <arnd@...db.de>, Avi Kivity <avi@...hat.com>,
	"Xin, Xiaohui" <xiaohui.xin@...el.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] macvtap: TX zero copy between guest and host
 kernel

On Tue, Sep 14, 2010 at 12:20:29PM -0700, Shirley Ma wrote:
> On Tue, 2010-09-14 at 21:01 +0200, Michael S. Tsirkin wrote:
> > On Tue, Sep 14, 2010 at 11:49:03AM -0700, Shirley Ma wrote:
> > > On Tue, 2010-09-14 at 20:27 +0200, Michael S. Tsirkin wrote:
> > > > As others said, the harder issues for TX are in determining that
> > it's
> > > > safe
> > > > to unpin the memory, and how much memory is it safe to pin to
> > beging
> > > > with.  For RX we have some more complexity.
> > > 
> > > I think unpin the memory is in kfree_skb() whenever the last
> > reference
> > > is gone for TX. What we discussed about here is when/how vhost get
> > > notified to update ring buffer descriptors. Do I misunderstand
> > something
> > > here? 
> > 
> > Right, that's a better way to put it. 
> 
> That's how this macvtap patch did. For how much pinned pages,it is
> limited by sk_wmem_alloc size in this patch.
> 
> thanks
> Shirley

Except that you seem to pin full pages but account sub-page size
in wmem.

-- 
MST
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