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 12:34:19 +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 9/30/2021 2:21 AM, Jason Gunthorpe wrote:
> On Thu, Sep 30, 2021 at 12:48:55AM +0300, Max Gurtovoy wrote:
>> 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.
> Yes, when a vfio_device first opens it must be running - ie able to do
> DMA and otherwise operational.

how can non resumed device do DMA ?

Also the bus master is not set.

>
> When we add the migration extension this cannot change, so after
> open_device() the device should be operational.

if it's waiting for incoming migration blob, it is not running.

>
> The reported state in the migration region should accurately reflect
> what the device is currently doing. If the device is operational then
> it must report running, not stopped.

STOP in migration meaning.

>
> Thus a driver cannot just zero initalize the migration "registers",
> they have to be accurate.
>
>>>> 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 ?
> It seems semantically fine to me, as per Alex's note what will happen
> is defined:
>
> driver will see RESUMING toggle off so it will trigger a
> de-serialization

You mean stop serialization ?

>
> driver will see SAVING toggled on so it will serialize the new state
> (either the pre-copy state or the post-copy state dpending on the
> running bit)

lets leave the bits and how you implement the state numbering aside.

If you finish resuming you can move to a new state (that we should add) 
=> RESUMED.

Now you suggested moving from RESUMED to SAVING to get the state again 
from the dst device ? and send it back to src ? before staring the VM 
and moving to RUNNING ?

where this is coming from ?

>
> Depending on the running bit the device may or may not be woken up.

lets take about logic here and not bits.

>
> If de-serialization fails then the state goes to error and SAVING is
> ignored.
>
> The driver logic probably looks something like this:
>
> // Running toggles off
> if (oldstate & RUNNING != newstate & RUNNING && oldstate & RUNNING)
>      queice
>      freeze
>
> // Resuming toggles off
> if (oldstate & RESUMING != newstate & RESUMING && oldstate & RESUMING)
>     deserialize
>
> // Saving toggles on
> if (oldstate & SAVING != newstate & SAVING && newstate & SAVING)
>     if (!(newstate & RUNNING))
>       serialize post copy
>
> // Running toggles on
> if (oldstate & RUNNING != newstate & RUNNING && newstate & RUNNING)
>     unfreeze
>     unqueice
>
> I'd have to check that carefully against the state chart from my last
> email though..
>
> And need to check how the "Stop Active Transactions" bit fits in there
>
> Jason