Re: [PATCH 2/2] nvme-multipath: enable PCI P2PDMA for multipath devices

On Tue, Mar 24, 2026 at 07:49:37AM +0100, Christoph Hellwig wrote:
> On Mon, Mar 23, 2026 at 04:44:16PM -0700, Chaitanya Kulkarni wrote:
> > diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> > index ba00f0b72b85..c49fca43ef19 100644
> > --- a/drivers/nvme/host/multipath.c
> > +++ b/drivers/nvme/host/multipath.c
> > @@ -737,6 +737,9 @@ int nvme_mpath_alloc_disk(struct nvme_ctrl *ctrl, struct nvme_ns_head *head)
> >  		BLK_FEAT_POLL | BLK_FEAT_ATOMIC_WRITES;
> >  	if (head->ids.csi == NVME_CSI_ZNS)
> >  		lim.features |= BLK_FEAT_ZONED;
> > +	if (ctrl->ops && ctrl->ops->supports_pci_p2pdma &&
> > +	    ctrl->ops->supports_pci_p2pdma(ctrl))
> > +		lim.features |= BLK_FEAT_PCI_P2PDMA;
> 
> This assumes all controllers support P2P, but we allow matching
> over different transports.  So you'll need to do the same scheme
> as for MD RAID that checks that every member supports P2P.

If that is a possible setup, then you could add a path that is non-P2P
capable sometime after the MD volume was setup with P2P supported, so
that case might need special handling to notify the stacking device of
the new limits.