netdev - Re: [PATCH mlx5-next 2/7] vfio: Add an API to check migration state transition validity

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Date:   Thu, 30 Sep 2021 14:01:43 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Max Gurtovoy <mgurtovoy@...dia.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Leon Romanovsky <leon@...nel.org>,
        Doug Ledford <dledford@...hat.com>,
        Yishai Hadas <yishaih@...dia.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Kirti Wankhede <kwankhede@...dia.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-rdma@...r.kernel.org, netdev@...r.kernel.org,
        Saeed Mahameed <saeedm@...dia.com>,
        Cornelia Huck <cohuck@...hat.com>
Subject: Re: [PATCH mlx5-next 2/7] vfio: Add an API to check migration state
 transition validity

On Thu, Sep 30, 2021 at 07:51:22PM +0300, Max Gurtovoy wrote:
> 
> On 9/30/2021 7:24 PM, Jason Gunthorpe wrote:
> > On Thu, Sep 30, 2021 at 06:32:07PM +0300, Max Gurtovoy wrote:
> > > > Just prior to open device the vfio pci layer will generate a FLR to
> > > > the function so we expect that post open_device has a fresh from reset
> > > > fully running device state.
> > > running also mean that the device doesn't have a clue on its internal state
> > > ? or running means unfreezed and unquiesced ?
> > The device just got FLR'd and it should be in a clean state and
> > operating. Think the VM is booting for the first time.
> 
> During the resume phase in the dst, the VM is paused and not booting.
> Migration SW is waiting to get memory and state from SRC. The device will
> start from the exact point that was in the src.
> 
> it's exactly "000b => Device Stopped, not saving or resuming"

For this case qmeu should open the VFIO device and immediately issue a
command to go to resuming. The kernel cannot know at open_device time
which case userspace is trying to do. Due to backwards compat we
assume userspace is going to boot a fresh VM.

> Well, this is your design for the driver implementation. Nobody is
> preventing other drivers to start deserializing device state into the device
> during RESUMING bit on.

It is a logical model. Devices can stream the migration data directly
into the internal state if they like. It just creates more conditions
where they have report an error state.

> So if we moved from 100b to 010b somehow, one should deserialized its buffer
> to the device, and then serialize it to migration region again ?

Yes.
 
> I guess its doable since the device is freeze and quiesced. But moving from
> 100b to 011b is not possible, right ?

Why not?

100b to 011b is no different than going indirectly 100b -> 001b -> 011b

The time spent in 001b is just negligable.

Jason