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

From: "Dong, Eddie" <eddie.dong@intel.com>
To: Christoph Hellwig <hch@lst.de>, Jason Gunthorpe <jgg@ziepe.ca>
Cc: "Rao, Lei" <lei.rao@intel.com>,
	"kbusch@kernel.org" <kbusch@kernel.org>,
	"axboe@fb.com" <axboe@fb.com>, "kch@nvidia.com" <kch@nvidia.com>,
	"sagi@grimberg.me" <sagi@grimberg.me>,
	"alex.williamson@redhat.com" <alex.williamson@redhat.com>,
	"cohuck@redhat.com" <cohuck@redhat.com>,
	"yishaih@nvidia.com" <yishaih@nvidia.com>,
	"shameerali.kolothum.thodi@huawei.com" 
	<shameerali.kolothum.thodi@huawei.com>,
	"Tian, Kevin" <kevin.tian@intel.com>,
	"mjrosato@linux.ibm.com" <mjrosato@linux.ibm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-nvme@lists.infradead.org" <linux-nvme@lists.infradead.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"Li, Yadong" <yadong.li@intel.com>,
	"Liu, Yi L" <yi.l.liu@intel.com>,
	"Wilk, Konrad" <konrad.wilk@oracle.com>,
	"stephen@eideticom.com" <stephen@eideticom.com>,
	"Yuan, Hang" <hang.yuan@intel.com>
Subject: RE: [RFC PATCH 5/5] nvme-vfio: Add a document for the NVMe device
Date: Tue, 6 Dec 2022 18:00:27 +0000	[thread overview]
Message-ID: <BL0PR11MB3042F27C99080E4B51318B338A1B9@BL0PR11MB3042.namprd11.prod.outlook.com> (raw)
In-Reply-To: <20221206153546.GA2266@lst.de>

> -----Original Message-----
> From: Christoph Hellwig <hch@lst.de>
> Sent: Tuesday, December 6, 2022 7:36 AM
> To: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: Christoph Hellwig <hch@lst.de>; Rao, Lei <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; Tian, Kevin <kevin.tian@intel.com>;
> mjrosato@linux.ibm.com; linux-kernel@vger.kernel.org; linux-
> nvme@lists.infradead.org; kvm@vger.kernel.org; Dong, Eddie
> <eddie.dong@intel.com>; Li, Yadong <yadong.li@intel.com>; Liu, Yi L
> <yi.l.liu@intel.com>; Wilk, Konrad <konrad.wilk@oracle.com>;
> stephen@eideticom.com; Yuan, Hang <hang.yuan@intel.com>
> Subject: Re: [RFC PATCH 5/5] nvme-vfio: Add a document for the NVMe device
> 
> On Tue, Dec 06, 2022 at 11:28:12AM -0400, Jason Gunthorpe wrote:
> > I'm interested as well, my mental model goes as far as mlx5 and
> > hisillicon, so if nvme prevents the VFs from being contained units, it
> > is a really big deviation from VFIO's migration design..
> 
> In NVMe the controller (which maps to a PCIe physical or virtual
> function) is unfortunately not very self contained.  A lot of state is subsystem-
> wide, where the subsystem is, roughly speaking, the container for all
> controllers that shared storage.  That is the right thing to do for say dual
> ported SSDs that are used for clustering or multi-pathing, for tentant isolation
> is it about as wrong as it gets.

NVMe spec is general, but the implementation details (such as internal state) may 
be vendor specific. If the migration happens between 2 identical NVMe devices 
(from same vendor/device w/ same firmware version), migration of 
subsystem-wide state can be naturally covered, right?

> 
> There is nothing in the NVMe spec that prohibits your from implementing
> multiple subsystems for multiple functions of a PCIe device, but if you do that
> there is absolutely no support in the spec to manage shared resources or any
> other interaction between them.

In IPU/DPU area, it seems multiple VFs with SR-IOV is widely adopted.

In VFs, the usage of shared resource can be viewed as implementation specific, 
and load/save state of a VF can rely on the hardware/firmware itself.
Migration of NVMe devices crossing vendor/device is another story: it may
be useful, but brings additional challenges. 