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

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Message-ID: <20221207183333.GA7049@lst.de>
Date:   Wed, 7 Dec 2022 19:33:33 +0100
From:   Christoph Hellwig <hch@....de>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Christoph Hellwig <hch@....de>, Lei Rao <lei.rao@...el.com>,
        kbusch@...nel.org, axboe@...com, kch@...dia.com, sagi@...mberg.me,
        alex.williamson@...hat.com, cohuck@...hat.com, yishaih@...dia.com,
        shameerali.kolothum.thodi@...wei.com, kevin.tian@...el.com,
        mjrosato@...ux.ibm.com, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, kvm@...r.kernel.org,
        eddie.dong@...el.com, yadong.li@...el.com, yi.l.liu@...el.com,
        Konrad.wilk@...cle.com, stephen@...eticom.com, hang.yuan@...el.com
Subject: Re: [RFC PATCH 1/5] nvme-pci: add function nvme_submit_vf_cmd to
 issue admin commands for VF driver.

On Wed, Dec 07, 2022 at 01:31:44PM -0400, Jason Gunthorpe wrote:
> > Sorry, I meant VF.  Your continued using of SR-IOV teminology now keeps
> > confusing my mind so much that I start mistyping things.
> 
> Well, what words do you want to use?

The same I've used through this whole thread:  controlling and
controlled function.

> So I don't think I've learned anything more about your objection.
> 
> "fundamentally broken" doesn't help

The objection is that:

 - in hardware fundamentally only the controlling funtion can
   control live migration features on the controlled function,
   because the controlled function is assigned to a VM which has
   control over it.
 - for the same reason there is no portable way to even find
   the controlling function from a controlled function, unless
   you want to equate PF = controlling and VF = controlled,
   and even that breaks down in some corner cases
 - if you want to control live migration from the controlled
   VM you need a new vfio subdriver for a function that has
   absolutely no new functionality itself related to live
   migration (quirks for bugs excepted).

So by this architecture you build a convoluted mess where you
need tons of vfio subdrivers that mostly just talk to the
driver for the controlling function, which they can't even
sanely discover.  That's what I keep calling fundamentally
broken.