Re: [RFC PATCH 1/5] nvme-pci: add function nvme_submit_vf_cmd to issue admin commands for VF driver.

[Jason Gunthorpe <jgg@ziepe.ca>]

On Tue, Dec 06, 2022 at 01:51:32PM +0000, Keith Busch wrote:
> On Tue, Dec 06, 2022 at 09:44:08AM -0400, Jason Gunthorpe wrote:
> > On Tue, Dec 06, 2022 at 07:19:40AM +0100, Christoph Hellwig wrote:
> > > On Tue, Dec 06, 2022 at 01:58:12PM +0800, Lei Rao wrote:
> > > > The new function nvme_submit_vf_cmd() helps the host VF driver to issue
> > > > VF admin commands. It's helpful in some cases that the host NVMe driver
> > > > does not control VF's admin queue. For example, in the virtualization
> > > > device pass-through case, the VF controller's admin queue is governed
> > > > by the Guest NVMe driver. Host VF driver relies on PF device's admin
> > > > queue to control VF devices like vendor-specific live migration commands.
> > > 
> > > WTF are you even smoking when you think this would be acceptable?
> > 
> > Not speaking to NVMe - but this driver is clearly copying mlx5's live
> > migration driver, almost completely - including this basic function.
> > 
> > So, to explain why mlx5 works this way..
> > 
> > The VFIO approach is to fully assign an entire VF to the guest OS. The
> > entire VF assignment means every MMIO register *and all the DMA* of
> > the VF is owned by the guest operating system.
> > 
> > mlx5 needs to transfer hundreds of megabytes to gigabytes of in-device
> > state to perform a migration.
> 
> For storage, though, you can't just transfer the controller state. You have to
> transfer all the namespace user data, too. So potentially many terabytes?

There are two different scenarios - lets call it shared medium and
local medium.

If the medium is shared then only the controller state needs to be
transfered. The controller state would include enough information to
locate and identify the shared medium.

This would apply to cases like DPU/smart NIC, multi-port physical
drives, etc.

local medium will require the medium copy. Within Linux I don't have a
clear sense if that should be done within the VFIO migration
framework, or if it would better to have its own operations.

For the NVMe spec I'd strongly suggest keeping medium copy as its own
set of commands.

It will be interesting to see how to standardize all these scenarios :)

Jason