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

[Michael S. Tsirkin]

On Thu, Apr 19, 2018 at 04:52:20PM +0000, Liang, Cunming wrote:
> 
> 
> > -----Original Message-----
> > From: Paolo Bonzini [mailto:address@hidden
> > Sent: Thursday, April 19, 2018 11:52 PM
> > To: Michael S. Tsirkin <address@hidden>; Liang, Cunming
> > <address@hidden>
> > Cc: Bie, Tiwei <address@hidden>; address@hidden;
> > address@hidden; address@hidden; address@hidden;
> > address@hidden; Daly, Dan <address@hidden>; Tan, Jianfeng
> > <address@hidden>; Wang, Zhihong <address@hidden>; Wang,
> > Xiao W <address@hidden>
> > 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