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 12:25:23 +0300
From:   Max Gurtovoy <mgurtovoy@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     Jason Gunthorpe <jgg@...pe.ca>, 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 1:44 AM, Alex Williamson wrote:
> On Thu, 30 Sep 2021 00:48:55 +0300
> Max Gurtovoy <mgurtovoy@...dia.com> 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.
>>
>> I migration SW is not involved, nobody will read this migration state.
> The _STOP state has a specific meaning regardless of whether userspace
> reads the device state value.  I think what you're suggesting is that
> the device reports itself as _STOP'd but it's actually _RUNNING.  Is
> that the compatibility workaround, create a self inconsistency?

 From migration point of view the device is stopped.

>
> We cannot impose on userspace to move a device from _STOP to _RUNNING
> simply because the device supports the migration region, nor should we
> report a device state that is inconsistent with the actual device state.

In this case we can think maybe moving to running during enabling the 
bus master..


>
>>>> 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...
> _RESUMING to _SAVING seems like a good way to test round trip migration
> without running the device to modify the state.  Potentially it's a
> means to update a saved device migration data stream to a newer format
> using an intermediate driver version.

what do you mean by "without running the device to modify the state." ?

did you describe a case where you migrate from source to dst and then 
back to source with a new migration data format ?

>
> If a driver is written such that it simply sees clearing the _RESUME
> bit as an indicator to de-serialize the data stream to the device, and
> setting the _SAVING flag as an indicator to re-serialize that data
> stream from the device, then this is just a means to make use of
> existing data paths.
>
> The uAPI specifies a means for drivers to reject a state change, but
> that risks failing to support a transition which might find mainstream
> use cases.  I don't think common code should be responsible for
> filtering out viable transitions.  Thanks,
>
> Alex
>