Re: [Qemu-devel] [PATCH v3 6/6] vhost-user: support registering external host notifiers

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Apr 19, 2018 at 04:52:20PM +0000, Liang, Cunming wrote:
> 
> 
> > -----Original Message-----
> > From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> > Sent: Thursday, April 19, 2018 11:52 PM
> > To: Michael S. Tsirkin <mst@redhat.com>; Liang, Cunming
> > <cunming.liang@intel.com>
> > Cc: Bie, Tiwei <tiwei.bie@intel.com>; jasowang@redhat.com;
> > alex.williamson@redhat.com; stefanha@redhat.com; qemu-devel@nongnu.org;
> > virtio-dev@lists.oasis-open.org; Daly, Dan <dan.daly@intel.com>; Tan, Jianfeng
> > <jianfeng.tan@intel.com>; Wang, Zhihong <zhihong.wang@intel.com>; Wang,
> > Xiao W <xiao.w.wang@intel.com>
> > Subject: Re: [PATCH v3 6/6] vhost-user: support registering external host
> > notifiers
> > 
> > On 19/04/2018 17:42, Michael S. Tsirkin wrote:
> > >> A compiler barrier is enough on strongly-ordered memory platform.
> > >> As it doesn't re-order store, PCI device won't see a stale index
> > >> value. But a weakly-ordered memory needs sfence.
> > >
> > > Oh you are right.
> > >
> > > So it's only needed for non-intel platforms or when packets are in WC
> > > memory then. And I don't know whether dpdk ever puts packets in WC
> > > memory.
> > >
> > > I guess we'll cross this bridge when we get to it.
> > 
> > Non-TSO architectures seem important...
> 
> I'm not familiar with Non-TSO, trying to understand the difference
> according to the feature set. Let's say non-TSO architectures do not
> set 'weak_barriers'. Then mandatory barrier is used for software. HW
> offload on that platform would choose different feature set against
> software? 

Right. We'll need a flag for this feature for starters. It doesn't exist
:) Paolo also points out that we should then add code to disallow
migration between setups with and without the feature.

> If it's not, essentially we're worried about live migration from a TSO to a non-TSO architectures platform?

Probably not.

> > 
> > Paolo