From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [RFC PATCH 2/2] macvtap: TX zero copy between guest and host kernel Date: Tue, 14 Sep 2010 11:12:23 +0200 Message-ID: <4C8F3C77.7010302@redhat.com> References: <1284410580.13351.10.camel@localhost.localdomain> <1284410883.13351.14.camel@localhost.localdomain> <20100913.201730.27805132.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: mashirle@us.ibm.com, arnd@arndb.de, mst@redhat.com, xiaohui.xin@intel.com, netdev@vger.kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: David Miller Return-path: Received: from mx1.redhat.com ([209.132.183.28]:61211 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752014Ab0INJMg (ORCPT ); Tue, 14 Sep 2010 05:12:36 -0400 In-Reply-To: <20100913.201730.27805132.davem@davemloft.net> Sender: kvm-owner@vger.kernel.org List-ID: On 09/14/2010 05:17 AM, David Miller wrote: > From: Shirley Ma > Date: Mon, 13 Sep 2010 13:48:03 -0700 > >> + base = (unsigned long)from->iov_base + offset1; >> + size = ((base& ~PAGE_MASK) + len + ~PAGE_MASK)>> PAGE_SHIFT; >> + num_pages = get_user_pages_fast(base, size, 0,&page[i]); >> + if ((num_pages != size) || >> + (num_pages> MAX_SKB_FRAGS - skb_shinfo(skb)->nr_frags)) >> + /* put_page is in skb free */ >> + return -EFAULT; > What keeps the user from writing to these pages in it's address space > after the write call returns? > > A write() return of success means: > > "I wrote what you gave to me" > > not > > "I wrote what you gave to me, oh and BTW don't touch these > pages for a while." > > In fact "a while" isn't even defined in any way, as there is no way > for the write() invoker to know when the networking card is done with > those pages. That's what io_submit() is for. Then io_getevents() tells you what "a while" actually was. -- error compiling committee.c: too many arguments to function