Re: [PATCH V6 4/6] hw/block/nvme: support for multi-controller in subsystem

[Minwoo Im <minwoo.im.dev@gmail.com>]

On 21-01-25 10:11:43, Keith Busch wrote:
> On Mon, Jan 25, 2021 at 07:03:32PM +0100, Klaus Jensen wrote:
> > On Jan 24 11:54, Minwoo Im wrote:
> > > We have nvme-subsys and nvme devices mapped together.  To support
> > > multi-controller scheme to this setup, controller identifier(id) has to
> > > be managed.  Earlier, cntlid(controller id) used to be always 0 because
> > > we didn't have any subsystem scheme that controller id matters.
> > > 
> > > This patch introduced 'cntlid' attribute to the nvme controller
> > > instance(NvmeCtrl) and make it allocated by the nvme-subsys device
> > > mapped to the controller.  If nvme-subsys is not given to the
> > > controller, then it will always be 0 as it was.
> > > 
> > > Added 'ctrls' array in the nvme-subsys instance to manage attached
> > > controllers to the subsystem with a limit(32).  This patch didn't take
> > > list for the controllers to make it seamless with nvme-ns device.
> > > 
> > > Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
> > > ---
> > >  hw/block/nvme-subsys.c | 21 +++++++++++++++++++++
> > >  hw/block/nvme-subsys.h |  4 ++++
> > >  hw/block/nvme.c        | 29 +++++++++++++++++++++++++++++
> > >  hw/block/nvme.h        |  1 +
> > >  4 files changed, 55 insertions(+)
> > > 
> > > diff --git a/hw/block/nvme.c b/hw/block/nvme.c
> > > index b525fca14103..7138389be4bd 100644
> > > --- a/hw/block/nvme.c
> > > +++ b/hw/block/nvme.c
> > > @@ -4481,6 +4484,10 @@ static void nvme_init_ctrl(NvmeCtrl *n, PCIDevice *pci_dev)
> > >      id->psd[0].enlat = cpu_to_le32(0x10);
> > >      id->psd[0].exlat = cpu_to_le32(0x4);
> > >  
> > > +    if (n->subsys) {
> > > +        id->cmic |= NVME_CMIC_MULTI_CTRL;
> > > +    }
> > 
> > Since multiple controllers show up with a PCIe port of their own, do we
> > need to set bit 0 (NVME_CMIC_MULTI_PORT?) as well? Or am I
> > misunderstanding that bit?
> 
> AIUI, if you report this MULTI_PORT bit, then each PCI device in the
> subsystem needs to report a different "Port Number" in their PCIe Link
> Capabilities register. I don't think we can manipulate that value from
> the nvme "device", but I also don't know what a host should do with this
> information even if we could. So, I think it's safe to leave it at 0.

AFAIK, If we leave it to 0, kernel will not allocate disk for multi-path
case (e.g., nvmeXcYnZ).