[RFC PATCH 5/5] nvme-vfio: Add a document for the NVMe device

[Rao, Lei]

On 12/11/2022 8:05 PM, Max Gurtovoy wrote:
> 
> On 12/6/2022 5:01 PM, Christoph Hellwig wrote:
>> On Tue, Dec 06, 2022 at 10:48:22AM -0400, Jason Gunthorpe wrote:
>>> Sadly in Linux we don't have a SRIOV VF lifecycle model that is any
>>> use.
>> Beward:  The secondary function might as well be a physical function
>> as well.  In fact one of the major customers for "smart" multifunction
>> nvme devices prefers multi-PF devices over SR-IOV VFs. (and all the
>> symmetric dual ported devices are multi-PF as well).
>>
>> So this isn't really about a VF live cycle, but how to manage life
>> migration, especially on the receive / restore side.  And restoring
>> the entire controller state is extremely invasive and can't be done
>> on a controller that is in any classic form live.  In fact a lot
>> of the state is subsystem-wide, so without some kind of virtualization
>> of the subsystem it is impossible to actually restore the state.
> 
> ohh, great !
> 
> I read this subsystem virtualization proposal of yours after I sent my proposal for subsystem virtualization in patch 1/5 thread.
> I guess this means that this is the right way to go.
> Lets continue brainstorming this idea. I think this can be the way to migrate NVMe controllers in a standard way.
> 
>>
>> To cycle back to the hardware that is posted here, I'm really confused
>> how it actually has any chance to work and no one has even tried
>> to explain how it is supposed to work.
> 
> I guess in vendor specific implementation you can assume some things that we are discussing now for making it as a standard.

Yes, as I wrote in the cover letter, this is a reference implementation to
start a discussion and help drive standardization efforts, but this series
works well for Intel IPU NVMe. As Jason said, there are two use cases:
shared medium and local medium. I think the live migration of the local medium
is complicated due to the large amount of user data that needs to be migrated.
I don't have a good idea to deal with this situation. But for Intel IPU NVMe,
each VF can connect to remote storage via the NVMF protocol to achieve storage
offloading. This is the shared medium. In this case, we don't need to migrate
the user data, which will significantly simplify the work of live migration.
The series tries to solve the problem of live migration of shared medium.
But it still lacks dirty page tracking and P2P support, we are also developing
these features.

About the nvme device state, As described in my document, the VF states include
VF CSR registers, Every IO Queue Pair state, and the AdminQ state. During the
implementation, I found that the device state data is small per VF. So, I decided
to use the admin queue of the Primary controller to send the live migration
commands to save and restore the VF states like MLX5.

Thanks,
Lei

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