Re: [RFC PATCH V2 00/11] hw/block/nvme: support multi-path for ctrl/ns

[Klaus Jensen <its@irrelevant.dk>]

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On Jan 17 23:53, Minwoo Im wrote:
> Hello,
> 
> This patch series introduces NVMe subsystem device to support multi-path
> I/O in NVMe device model.  Two use-cases are supported along with this
> patch: Multi-controller, Namespace Sharing.
> 
> V1 RFC has been discussed with Klaus and Keith, I really appreciate them
> for this patch series to have proper direction [1].
> 
> This patch series contains few start-up refactoring pathces from the
> first to fifth patches to make nvme-ns device not to rely on the nvme
> controller always.  Because nvme-ns shall be able to be mapped to the
> subsystem level, not a single controller level so that it should provide
> generic initialization code: nvme_ns_setup() with NvmeCtrl.  To do that,
> the first five patches are to remove the NvmeCtrl * instance argument
> from the nvme_ns_setup().  I'd be happy if they are picked!
> 
> For controller and namespace devices, 'subsys' property has been
> introduced to map them to a subsystem.  If multi-controller needed, we
> can specify 'subsys' to controllers the same.
> 
> For namespace deivice, if 'subsys' is not given just like it was, it
> will have to be provided with 'bus' parameter to specify a nvme
> controller device to attach, it means, they are mutual-exlusive.  To
> share a namespace between or among controllers, then nvme-ns should have
> 'subsys' property to a single nvme subsystem instance.  To make a
> namespace private one, then we need to specify 'bus' property rather
> than the 'subsys'.
> 
> Of course, this series does not require any updates for the run command
> for the previos users.
> 
> Plase refer the following example with nvme-cli output:
> 
> QEMU Run:
>   -device nvme-subsys,id=subsys0 \
>   -device nvme,serial=foo,id=nvme0,subsys=subsys0 \
>   -device nvme,serial=bar,id=nvme1,subsys=subsys0 \
>   -device nvme,serial=baz,id=nvme2,subsys=subsys0 \
>   -device nvme-ns,id=ns1,drive=drv10,nsid=1,subsys=subsys0 \
>   -device nvme-ns,id=ns2,drive=drv11,nsid=2,bus=nvme2 \
>   \
>   -device nvme,serial=qux,id=nvme3 \
>   -device nvme-ns,id=ns3,drive=drv12,nsid=3,bus=nvme3
> 
> nvme-cli:
>   root@vm:~/work# nvme list -v
>   NVM Express Subsystems
> 
>   Subsystem        Subsystem-NQN                                                                                    Controllers
>   ---------------- ------------------------------------------------------------------------------------------------ ----------------
>   nvme-subsys1     nqn.2019-08.org.qemu:subsys0                                                                     nvme0, nvme1, nvme2
>   nvme-subsys3     nqn.2019-08.org.qemu:qux                                                                         nvme3
> 
>   NVM Express Controllers
> 
>   Device   SN                   MN                                       FR       TxPort Address        Subsystem    Namespaces
>   -------- -------------------- ---------------------------------------- -------- ------ -------------- ------------ ----------------
>   nvme0    foo                  QEMU NVMe Ctrl                           1.0      pcie   0000:00:06.0   nvme-subsys1 nvme1n1
>   nvme1    bar                  QEMU NVMe Ctrl                           1.0      pcie   0000:00:07.0   nvme-subsys1 nvme1n1
>   nvme2    baz                  QEMU NVMe Ctrl                           1.0      pcie   0000:00:08.0   nvme-subsys1 nvme1n1, nvme1n2
>   nvme3    qux                  QEMU NVMe Ctrl                           1.0      pcie   0000:00:09.0   nvme-subsys3
> 
>   NVM Express Namespaces
> 
>   Device       NSID     Usage                      Format           Controllers
>   ------------ -------- -------------------------- ---------------- ----------------
>   nvme1n1      1        134.22  MB / 134.22  MB    512   B +  0 B   nvme0, nvme1, nvme2
>   nvme1n2      2        268.44  MB / 268.44  MB    512   B +  0 B   nvme2
>   nvme3n1      3        268.44  MB / 268.44  MB    512   B +  0 B   nvme3
> 
> Summary:
>   - Refactored nvme-ns device not to rely on controller during the
>     setup.  [1/11 - 5/11]
>   - Introduced a nvme-subsys device model. [6/11]
>   - Create subsystem NQN based on subsystem. [7/11]
>   - Introduced multi-controller model. [8/11 - 9/11]
>   - Updated namespace sharing scheme to be based on nvme-subsys
>     hierarchy. [10/11 - 11/11]
> 
> Since RFC V1:
>   - Updated namespace sharing scheme to be based on nvme-subsys
>     hierarchy.
> 

Great stuff Minwoo. Thanks!

I'll pick up [01-05/11] directly since they are pretty trivial.

The subsystem model looks pretty much like it should, I don't have a lot
of comments.

One thing that I considered, is if we should reverse the "registration"
and think about it as namespace attachment. The spec is about
controllers attaching to namespaces, not the other way around.
Basically, let the namespaces be configured first and register on the
subsystem (accumulating in a "namespaces" array), then have the
controllers register with the subsystem and attach to all "non-detached"
namespaces. This allows detached namespaces to "linger" in the subsystem
to be attached later on. If there are any private namespaces (like ns2
in your example above), it will be defined after the controller with the
bus=ctrlX parameter like normal.

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