linux-kernel - Re: remove the nvlink2 pci

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Message-ID: <20210504155327.GA94750@nvidia.com>
Date:   Tue, 4 May 2021 12:53:27 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Greg Kurz <groug@...d.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christoph Hellwig <hch@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Alex Williamson <alex.williamson@...hat.com>,
        kvm@...r.kernel.org, David Airlie <airlied@...ux.ie>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Paul Mackerras <paulus@...ba.org>, linux-api@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, qemu-devel@...gnu.org,
        qemu-ppc@...gnu.org
Subject: Re: remove the nvlink2 pci_vfio subdriver v2

On Tue, May 04, 2021 at 04:23:40PM +0200, Daniel Vetter wrote:

> Just my 2cents from drm (where we deprecate old gunk uapi quite often):
> Imo it's best to keep the uapi headers as-is, but exchange the
> documentation with a big "this is removed, never use again" warning:

We in RDMA have been doing the opposite, the uapi headers are supposed
to reflect the current kernel. This helps make the kernel
understandable.

When userspace needs backwards compat to ABI that the current kernel
doesn't support then userspace has distinct copies of that information
in some compat location. It has happened a few times over the last 15
years.

We keep full copies of the current kernel headers in the userspace
source tree, when the kernel headers change in a compile incompatible
way we fix everything while updating to the new kernel headers.

> - it's good to know which uapi numbers (like parameter extensions or
>   whatever they are in this case) are defacto reserved, because there are
>   binaries (qemu in this) that have code acting on them out there.

Numbers and things get marked reserved or the like

> Anyway feel free to ignore since this might be different than drivers/gpu.

AFAIK drives/gpu has a lot wider userspace, rdma manages this OK
because we only have one library package that provides the user/kernel
interface.

Jason