linux-kernel - Re: [PATCH v8 1/1] vfio/nvgpu: Add vfio pci variant module for grace hopper

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Message-ID: <ZPsQf9pGrSnbFI8p@nvidia.com>
Date:   Fri, 8 Sep 2023 09:15:59 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Alex Williamson <alex.williamson@...hat.com>, ankita@...dia.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, anuaggarwal@...dia.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 1/1] vfio/nvgpu: Add vfio pci variant module for grace
 hopper

On Fri, Sep 08, 2023 at 01:50:36AM -0700, Christoph Hellwig wrote:
> On Thu, Sep 07, 2023 at 10:04:10PM -0600, Alex Williamson wrote:
> > > I think it really depends on what the qemu side wants to do..
> >
> > Ok, I thought you had been one of the proponents of the fake BAR
> > approach as more resembling CXL.  

Yes, I do prefer this because it is ultimately simpler on the qemu and
VM side. This ACPI tinkering is not nice.

> > Do we need to reevaluate that the tinkering with the VM machine
> > topology and firmware tables would better align to a device
> > specific region that QEMU inserts into the VM address space so
> > that bare metal and virtual machine versions of this device look
> > more similar?  Thanks,

> Yes, providing something to a VM that doesn't look anything like the
> underlying hardware feels pretty strange.

I don't see the goal as perfect emulation of the real HW.

Aiming for minimally disruptive to the ecosystem to support this
quirky pre-CXL HW.

Perfect emulation would need a unique VFIO uAPI and more complex qemu
changes, and it really brings nothing of value.

Jason