linux-kernel - Re: [PATCHv4 2/2] vhost_net: a kernel-level virtio server

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Message-ID: <20090827111947.GA21801@redhat.com>
Date:	Thu, 27 Aug 2009 14:19:48 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, mingo@...e.hu,
	linux-mm@...ck.org, akpm@...ux-foundation.org, hpa@...or.com,
	gregory.haskins@...il.com
Subject: Re: [PATCHv4 2/2] vhost_net: a kernel-level virtio server

On Thu, Aug 27, 2009 at 08:40:02PM +0930, Rusty Russell wrote:
> On Thu, 27 Aug 2009 08:15:18 pm Michael S. Tsirkin wrote:
> > On Tue, Aug 25, 2009 at 09:40:40PM +0930, Rusty Russell wrote:
> > > Also, see other fixes to the lguest launcher since then which might
> > > be relevant to this code:
> > > 	lguest: get more serious about wmb() in example Launcher code
> > 
> > Heh, this just gets one step closer to a real wmb.  I just used the
> > correct code from linux, so I think nothing needs to be done in vhost.
> > Apropos this change in lguest: why is a compiler barrier sufficient? The
> > comment says devices are run in separate threads (presumably from
> > guest?), if so don't you need to tell CPU that there's a barrier as
> > well?
> 
> Yep, but x86 only :)

Okay, writes are ordered them. But mb() orders reads as well,
so it'll have to have a real one IMO, even in userspace?

>  The kernel uses a real insn if XMM/XMM2, but I don't
> know if userspace needs that.  I just use compiler barriers.
> 
> Thanks,
> Rusty.
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