Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Received-SPF: Pass (protection.outlook.com: domain of nvidia.com designates
 216.228.112.34 as permitted sender) receiver=protection.outlook.com;
 client-ip=216.228.112.34; helo=mail.nvidia.com;
Subject: Re: [PATCH mlx5-next 2/7] vfio: Add an API to check migration state
 transition validity
To:     Jason Gunthorpe <jgg@ziepe.ca>
CC:     Alex Williamson <alex.williamson@redhat.com>,
        Leon Romanovsky <leon@kernel.org>,
        Doug Ledford <dledford@redhat.com>,
        Yishai Hadas <yishaih@nvidia.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Kirti Wankhede <kwankhede@nvidia.com>, <kvm@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <linux-pci@vger.kernel.org>,
        <linux-rdma@vger.kernel.org>, <netdev@vger.kernel.org>,
        Saeed Mahameed <saeedm@nvidia.com>,
        Cornelia Huck <cohuck@redhat.com>
References: <20210927231239.GE3544071@ziepe.ca>
 <25c97be6-eb4a-fdc8-3ac1-5628073f0214@nvidia.com>
 <20210929063551.47590fbb.alex.williamson@redhat.com>
 <1eba059c-4743-4675-9f72-1a26b8f3c0f6@nvidia.com>
 <20210929075019.48d07deb.alex.williamson@redhat.com>
 <d2e94241-a146-c57d-cf81-8b7d8d00e62d@nvidia.com>
 <20210929091712.6390141c.alex.williamson@redhat.com>
 <e1ba006f-f181-0b89-822d-890396e81c7b@nvidia.com>
 <20210929161433.GA1808627@ziepe.ca>
 <29835bf4-d094-ae6d-1a32-08e65847b52c@nvidia.com>
 <20210929232109.GC3544071@ziepe.ca>
From:   Max Gurtovoy <mgurtovoy@nvidia.com>
Message-ID: <d8324d96-c897-b914-16c6-ad0bbb9b13a5@nvidia.com>
Date:   Thu, 30 Sep 2021 12:34:19 +0300
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101
 Thunderbird/78.14.0
MIME-Version: 1.0
In-Reply-To: <20210929232109.GC3544071@ziepe.ca>
Content-Type: text/plain; charset="utf-8"; format=flowed
Content-Transfer-Encoding: 7bit
Content-Language: en-US
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 30 Sep 2021 09:34:26.6271
 (UTC)
X-MS-Exchange-CrossTenant-Network-Message-Id: 9bb305a1-0fde-4212-7f44-08d983f57ece
X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a
X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.112.34];Helo=[mail.nvidia.com]
X-MS-Exchange-CrossTenant-AuthSource: CO1NAM11FT054.eop-nam11.prod.protection.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Anonymous
X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem
X-MS-Exchange-Transport-CrossTenantHeadersStamped: BN6PR1201MB2528
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


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