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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Date:   Tue, 6 Dec 2022 13:51:32 +0000
From:   Keith Busch <kbusch@...nel.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Christoph Hellwig <hch@....de>, Lei Rao <lei.rao@...el.com>,
        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 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?