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Date:   Tue, 6 Dec 2022 10:20:26 -0400
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Lei Rao <lei.rao@intel.com>, kbusch@kernel.org, axboe@fb.com,
        kch@nvidia.com, sagi@grimberg.me, alex.williamson@redhat.com,
        cohuck@redhat.com, yishaih@nvidia.com,
        shameerali.kolothum.thodi@huawei.com, kevin.tian@intel.com,
        mjrosato@linux.ibm.com, linux-kernel@vger.kernel.org,
        linux-nvme@lists.infradead.org, kvm@vger.kernel.org,
        eddie.dong@intel.com, yadong.li@intel.com, yi.l.liu@intel.com,
        Konrad.wilk@oracle.com, stephen@eideticom.com, hang.yuan@intel.com
Subject: Re: [RFC PATCH 5/5] nvme-vfio: Add a document for the NVMe device
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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