netdev - RE: [PATCH V7 mlx5-next 08/15] vfio: Define device migration protocol v2

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Date:   Tue, 15 Feb 2022 10:58:58 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>
CC:     Yishai Hadas <yishaih@...dia.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 08/15] vfio: Define device migration protocol
 v2

> From: Tian, Kevin
> Sent: Tuesday, February 15, 2022 6:42 PM
> 
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Wednesday, February 9, 2022 10:37 AM
> >
> > > >  /* -------- API for Type1 VFIO IOMMU -------- */
> > > >
> > > >  /**
> > >
> > > Otherwise, I'm still not sure how userspace handles the fact that it
> > > can't know how much data will be read from the device and how
> important
> > > that is.  There's no replacement of that feature from the v1 protocol
> > > here.
> >
> > I'm not sure this was part of the v1 protocol either. Yes it had a
> > pending_bytes, but I don't think it was actually expected to be 100%
> > accurate. Computing this value accurately is potentially quite
> > expensive, I would prefer we not enforce this on an implementation
> > without a reason, and qemu currently doesn't make use of it.
> >
> > The ioctl from the precopy patch is probably the best approach, I
> > think it would be fine to allow that for stop copy as well, but also
> > don't see a usage right now.
> >
> > It is not something that needs decision now, it is very easy to detect
> > if an ioctl is supported on the data_fd at runtime to add new things
> > here when needed.
> >
> 
> Another interesting thing (not an immediate concern on this series)
> is how to handle devices which may have long time (e.g. due to
> draining outstanding requests, even w/o vPRI) to enter the STOP
> state. that time is not as deterministic as pending bytes thus cannot
> be reported back to the user before the operation is actually done.
> 
> Similarly to what we discussed for vPRI an eventfd will be beneficial
> so the user can timeout-wait on it, but it also needs an arc to create
> the eventfd between RUNNING->STOP...
> 

type too fast. it doesn’t need a new arc. Just a new capability to say
that STOP returns an event fd for the user to wait for completion,
when supporting such devices is required. 😊

Thanks
Kevin