linux-kernel - Re: [PATCH v7 1/1] vfio/nvgpu: Add vfio pci variant module for grace hopper

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Date:   Wed, 30 Aug 2023 12:39:05 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     ankita@...dia.com, alex.williamson@...hat.com, yishaih@...dia.com,
        shameerali.kolothum.thodi@...wei.com, kevin.tian@...el.com,
        aniketa@...dia.com, cjia@...dia.com, kwankhede@...dia.com,
        targupta@...dia.com, vsethi@...dia.com, acurrid@...dia.com,
        apopple@...dia.com, jhubbard@...dia.com, danw@...dia.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Airlie <airlied@...hat.com>,
        dri-devel@...ts.freedesktop.org, Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v7 1/1] vfio/nvgpu: Add vfio pci variant module for grace
 hopper

On Wed, Aug 30, 2023 at 06:50:32AM -0700, Christoph Hellwig wrote:
> I know I'm chiming in a bit late, but what ultimate user space is going
> to use this?  We should not add anything to the kernel that can't
> be used without fully open user space.

qemu will get the matching VFIO userspace patches, I think they were
posted someplace already.

> vfio has traditionally been a bit special as it "just" passes devices
> through, so any user space could just be a user space driver for a
> random device on $FOO bus, including an actual Linux driver in a VM,
> but this driver has very specific semantics for a very specific piece
> of hardware, so it really needs to be treated like a generic GPU driver
> or accelerator driver.

This is basically a pre-CXL driver. It takes a PCI device and some
non-standard CXL-ish metadata and adapts it to VFIO.

In a post-CXL world this same functionality of managing the 'cache
coherent BAR' for VFIO would be done generically by some generic
vfio-cxl driver.

Jason