netdev - Re: [PATCH V7 mlx5-next 15/15] vfio: Extend the device migration protocol with PRE

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Date:   Tue, 22 Feb 2022 20:44:32 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Yishai Hadas <yishaih@...dia.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "saeedm@...dia.com" <saeedm@...dia.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "leonro@...dia.com" <leonro@...dia.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "mgurtovoy@...dia.com" <mgurtovoy@...dia.com>,
        "maorg@...dia.com" <maorg@...dia.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>
Subject: Re: [PATCH V7 mlx5-next 15/15] vfio: Extend the device migration
 protocol with PRE_COPY

On Wed, Feb 23, 2022 at 12:40:58AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Tuesday, February 22, 2022 11:51 PM
> > 
> > On Tue, Feb 22, 2022 at 01:43:13AM +0000, Tian, Kevin wrote:
> > 
> > > > > > + * Drivers should attempt to return estimates so that initial_bytes +
> > > > > > + * dirty_bytes matches the amount of data an immediate transition
> > to
> > > > > > STOP_COPY
> > > > > > + * will require to be streamed.
> > > > >
> > > > > I didn't understand this requirement. In an immediate transition to
> > > > > STOP_COPY I expect the amount of data covers the entire device
> > > > > state, i.e. initial_bytes. dirty_bytes are dynamic and iteratively returned
> > > > > then why we need set some expectation on the sum of
> > > > > initial+round1_dity+round2_dirty+...
> > > >
> > > > "will require to be streamed" means additional data from this point
> > > > forward, not including anything already sent.
> > > >
> > > > It turns into the estimate of how long STOP_COPY will take.
> > >
> > > I still didn't get the 'match' part. Why should the amount of data which
> > > has already been sent match the additional data to be sent in STOP_COPY?
> > 
> > None of it is 'already been sent' the return values are always 'still
> > to be sent'
> > 
> 
> Reread the description:
> 
> + * Drivers should attempt to return estimates so that initial_bytes +
> + * dirty_bytes matches the amount of data an immediate transition to STOP_COPY
> + * will require to be streamed.
> 
> I guess you intended to mean that when EITHER initial_bytes OR
> dirty_bytes is read the returned value should match the amount 
> of data as described above. It is "+" which confused me to think 
> it as a sum of both numbers...

It is the sum

initial_bytes declines as the data is transferred. Once everything is
read out the sum will be 0.

Jason