linux-kernel - Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <BANLkTim9RQ94MT4L0QHx-yzZT5nXdbi0aQ@mail.gmail.com>
Date:	Wed, 18 May 2011 13:47:33 +0200
From:	Michał Mirosław <mirqus@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Shirley Ma <mashirle@...ibm.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Avi Kivity <avi@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice

W dniu 18 maja 2011 13:17 użytkownik Michael S. Tsirkin
<mst@...hat.com> napisał:
> On Wed, May 18, 2011 at 01:10:50PM +0200, Michał Mirosław wrote:
>> 2011/5/18 Michael S. Tsirkin <mst@...hat.com>:
>> > On Tue, May 17, 2011 at 03:28:38PM -0700, Shirley Ma wrote:
>> >> On Tue, 2011-05-17 at 23:48 +0200, Michał Mirosław wrote:
>> >> > 2011/5/17 Shirley Ma <mashirle@...ibm.com>:
>> >> > > Hello Michael,
>> >> > >
>> >> > > Looks like to use a new flag requires more time/work. I am thinking
>> >> > > whether we can just use HIGHDMA flag to enable zero-copy in macvtap
>> >> > to
>> >> > > avoid the new flag for now since mavctap uses real NICs as lower
>> >> > device?
>> >> >
>> >> > Is there any other restriction besides requiring driver to not recycle
>> >> > the skb? Are there any drivers that recycle TX skbs?
>> > Not just recycling skbs, keeping reference to any of the pages in the
>> > skb. Another requirement is to invoke the callback
>> > in a timely fashion.  For example virtio-net doesn't limit the time until
>> > that happens (skbs are only freed when some other packet is
>> > transmitted), so we need to avoid zcopy for such (nested-virt)
>> > scenarious, right?
>> Hmm. But every hardware driver supporting SG will keep reference to
>> the pages until the packet is sent (or DMA'd to the device). This can
>> take a long time if hardware queue happens to stall for some reason.
> That's a fundamental property of zero copy transmit.
> You can't let the application/guest reuse the memory until
> no one looks at it anymore.

One more question: is userspace (or whatever is sending those packets)
denied from modifying passed pages? I assume it is, but just want to
be sure.

Best Regards,
Michał Mirosław
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/