netdev - RE: [PATCH iwl-next v4 08/12] ice: Save and load RX Queue head

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Date: Fri, 8 Dec 2023 02:53:44 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: "Cao, Yahui" <yahui.cao@...el.com>, "intel-wired-lan@...ts.osuosl.org"
	<intel-wired-lan@...ts.osuosl.org>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"Liu, Lingyu" <lingyu.liu@...el.com>, "Chittim, Madhu"
	<madhu.chittim@...el.com>, "Samudrala, Sridhar"
	<sridhar.samudrala@...el.com>, "alex.williamson@...hat.com"
	<alex.williamson@...hat.com>, "yishaih@...dia.com" <yishaih@...dia.com>,
	"shameerali.kolothum.thodi@...wei.com"
	<shameerali.kolothum.thodi@...wei.com>, "brett.creeley@....com"
	<brett.creeley@....com>, "davem@...emloft.net" <davem@...emloft.net>,
	"edumazet@...gle.com" <edumazet@...gle.com>, "kuba@...nel.org"
	<kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>
Subject: RE: [PATCH iwl-next v4 08/12] ice: Save and load RX Queue head

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Thursday, December 7, 2023 10:46 PM
> 
> On Thu, Dec 07, 2023 at 07:55:17AM +0000, Tian, Kevin wrote:
> > > From: Cao, Yahui <yahui.cao@...el.com>
> > > Sent: Tuesday, November 21, 2023 10:51 AM
> > >
> > > +
> > > +		/* Once RX Queue is enabled, network traffic may come in at
> > > any
> > > +		 * time. As a result, RX Queue head needs to be loaded
> > > before
> > > +		 * RX Queue is enabled.
> > > +		 * For simplicity and integration, overwrite RX head just after
> > > +		 * RX ring context is configured.
> > > +		 */
> > > +		if (msg_slot->opcode == VIRTCHNL_OP_CONFIG_VSI_QUEUES)
> > > {
> > > +			ret = ice_migration_load_rx_head(vf, devstate);
> > > +			if (ret) {
> > > +				dev_err(dev, "VF %d failed to load rx head\n",
> > > +					vf->vf_id);
> > > +				goto out_clear_replay;
> > > +			}
> > > +		}
> > > +
> >
> > Don't we have the same problem here as for TX head restore that the
> > vfio migration protocol doesn't carry a way to tell whether the IOAS
> > associated with the device has been restored then allowing RX DMA
> > at this point might cause device error?
> 
> Does this trigger a DMA?

looks yes from the comment

> 
> > @Jason, is it a common gap applying to all devices which include a
> > receiving path from link? How is it handled in mlx migration
> > driver?
> 
> There should be no DMA until the device is placed in RUNNING. All
> devices may instantly trigger DMA once placed in RUNNING.
> 
> The VMM must ensure the entire environment is ready to go before
> putting anything in RUNNING, including having setup the IOMMU.
> 

ah, yes. that is the right behavior.

so if there is no other way to block DMA before RUNNING is reached,
here the RX queue should be left disabled until when transitioning 
to RUNNING.

Yahui, can you double check?