netdev - RE: [PATCH v10 vfio 4/7] vfio/pds: Add VFIO live migration support

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Date: Tue, 27 Jun 2023 06:03:29 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Brett Creeley <brett.creeley@....com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"alex.williamson@...hat.com" <alex.williamson@...hat.com>,
	"yishaih@...dia.com" <yishaih@...dia.com>,
	"shameerali.kolothum.thodi@...wei.com"
	<shameerali.kolothum.thodi@...wei.com>, "shannon.nelson@....com"
	<shannon.nelson@....com>
Subject: RE: [PATCH v10 vfio 4/7] vfio/pds: Add VFIO live migration support

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Tuesday, June 27, 2023 2:14 AM
> 
> On Mon, Jun 26, 2023 at 07:31:31AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > Sent: Wednesday, June 21, 2023 9:27 PM
> > >
> > > On Wed, Jun 21, 2023 at 06:49:12AM +0000, Tian, Kevin wrote:
> > >
> > > > What is the criteria for 'reasonable'? How does CSPs judge that such
> > > > device can guarantee a *reliable* reasonable window so live migration
> > > > can be enabled in the production environment?
> > >
> > > The CSP needs to work with the device vendor to understand how it fits
> > > into their system, I don't see how we can externalize this kind of
> > > detail in a general way.
> > >
> > > > I'm afraid that we are hiding a non-deterministic factor in current
> protocol.
> > >
> > > Yes
> > >
> > > > But still I don't think it's a good situation where the user has ZERO
> > > > knowledge about the non-negligible time in the stopping path...
> > >
> > > In any sane device design this will be a small period of time. These
> > > timeouts should be to protect against a device that has gone wild.
> > >
> >
> > Any example how 'small' it will be (e.g. <1ms)?
> 
> Not personally..
> 
> > Should we define a *reasonable* threshold in VFIO community which
> > any new variant driver should provide information to judge against?
> 
> Ah, I think we are just too new to get into such details. I think we
> need some real world experience to see if this is really an issue.
> 
> > The reason why I keep discussing it is that IMHO achieving negligible
> > stop time is a very challenging task for many accelerators. e.g. IDXD
> > can be stopped only after completing all the pending requests. While
> > it allows software to configure the max pending work size (and a
> > reasonable setting could meet both migration SLA and performance
> > SLA) the worst-case draining latency could be in 10's milliseconds which
> > cannot be ignored by the VMM.
> 
> Well, what would you report here if you had the opportunity to report
> something? Some big number? Then what?
> 
> > Or do you think it's still better left to CSP working with the device vendor
> > even in this case, given the worst-case latency could be affected by
> > many factors hence not something which a kernel driver can accurately
> > estimate?
> 
> This is my fear, that it is so complicated that reducing it to any
> sort of cross-vendor data is not feasible. At least I'd like to see
> someone experiment with what information would be useful to qemu
> before we add kernel ABI..
> 

OK. make sense.