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

[Liang, Cunming]

> -----Original Message-----
> From: Michael S. Tsirkin [mailto:address@hidden
> Sent: Friday, April 20, 2018 1:01 AM
> To: Liang, Cunming <address@hidden>
> Cc: Paolo Bonzini <address@hidden>; 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 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.
I see. Thanks.

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