linux-kernel - 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 00:48:55 +0300
From:   Max Gurtovoy <mgurtovoy@...dia.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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 9/29/2021 7:14 PM, Jason Gunthorpe wrote:
> On Wed, Sep 29, 2021 at 06:28:44PM +0300, Max Gurtovoy wrote:
>
>>> So you have a device that's actively modifying its internal state,
>>> performing I/O, including DMA (thereby dirtying VM memory), all while
>>> in the _STOP state?  And you don't see this as a problem?
>> I don't see how is it different from vfio-pci situation.
> vfio-pci provides no way to observe the migration state. It isn't
> "000b"

Alex said that there is a problem of compatibility.

I migration SW is not involved, nobody will read this migration state.

>> Maybe we need to rename STOP state. We can call it READY or LIVE or
>> NON_MIGRATION_STATE.
> It was a poor choice to use 000b as stop, but it doesn't really
> matter. The mlx5 driver should just pre-init this readable to running.

I guess we can do it for this reason. There is no functional problem nor 
compatibility issue here as was mentioned.

But still we need the kernel to track transitions. We don't want to 
allow moving from RESUMING to SAVING state for example. How this 
transition can be allowed ?

In this case we need to fail the request from the migration SW...


>
> Jason