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

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Date:   Tue, 6 Dec 2022 10:20:26 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Christoph Hellwig <hch@....de>
Cc:     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 5/5] nvme-vfio: Add a document for the NVMe device

On Tue, Dec 06, 2022 at 03:00:02PM +0100, Christoph Hellwig wrote:
> > >From what I understood this series basically allows two Intel devices
> > to pass a big opaque blob of data. Intel didn't document what is in
> > that blob, so I assume it captures everything you mention above.
> 
> Which would be just as bad, because it then changes the IDs under
> the live OS on a restore.  This is not something that can be done
> behind the back of the hypervisors / control plane OS.

Sorry, what live OS?

In the VFIO restore model there is no "live OS" on resume. The
load/resume cycle is as destructive as reset to the vfio device.

When qemu operates vfio the destination CPU will not be running until
the load/resume of all the VFIO devices is completed.

So from the VM perspective it sees a complete no change, so long as
the data blob causes the destination vfio device to fully match the
source, including all IDs, etc.

Jason