linux-kernel - Re: [PATCH v12 1/1] vfio/nvgpu: Add vfio pci variant module for grace hopper

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Date:   Wed, 25 Oct 2023 08:29:47 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Ankit Agrawal <ankita@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Yishai Hadas <yishaih@...dia.com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        Aniket Agashe <aniketa@...dia.com>, Neo Jia <cjia@...dia.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        "Tarun Gupta (SW-GPU)" <targupta@...dia.com>,
        Vikram Sethi <vsethi@...dia.com>,
        "Currid, Andy" <acurrid@...dia.com>,
        Alistair Popple <apopple@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        Dan Williams <danw@...dia.com>,
        "Anuj Aggarwal (SW-GPU)" <anuaggarwal@...dia.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v12 1/1] vfio/nvgpu: Add vfio pci variant module for
 grace hopper

On Wed, 25 Oct 2023 08:28:44 +0000
"Tian, Kevin" <kevin.tian@...el.com> wrote:

> > From: Alex Williamson <alex.williamson@...hat.com>
> > Sent: Tuesday, October 24, 2023 10:29 PM
> > 
> > On Tue, 24 Oct 2023 14:03:25 +0000
> > Ankit Agrawal <ankita@...dia.com> wrote:
> >   
> > > >> > After looking at Yishai's virtio-vfio-pci driver where BAR0 is emulated
> > > >> > as an IO Port BAR, it occurs to me that there's no config space
> > > >> > emulation of BAR2 (or BAR3) here.  Doesn't this mean that QEMU  
> > registers  
> > > >> > the BAR as 32-bit, non-prefetchable?  ie. VFIOBAR.type & .mem64 are
> > > >> > wrong?  
> > > >>
> > > >> Maybe I didn't understand the question, but the PCI config space  
> > read/write  
> > > >> would still be handled by vfio_pci_core_read/write() which returns the
> > > >> appropriate flags. I have checked that the device BARs are 64b and
> > > >> prefetchable in the VM.  
> > > >
> > > > vfio_pci_core_read/write() accesses the physical device, which doesn't
> > > > implement BAR2.  Why would an unimplemented BAR2 on the physical  
> > device  
> > > > report 64-bit, prefetchable?
> > > >
> > > > QEMU records VFIOBAR.type and .mem64 from reading the BAR register  
> > in  
> > > > vfio_bar_prepare() and passes this type to pci_register_bar() in
> > > > vfio_bar_register().  Without an implementation of a config space read
> > > > op in the variant driver and with no physical implementation of BAR2 on
> > > > the device, I don't see how we get correct values in these fields.  
> > >
> > > I think I see the cause of confusion. There are real PCIe compliant BARs
> > > present on the device, just that it isn't being used once the C2C
> > > interconnect is active. The BARs are 64b prefetchable. Here it the lspci
> > > snippet of the device on the host.
> > > # lspci -v -s 9:1:0.0
> > > 0009:01:00.0 3D controller: NVIDIA Corporation Device 2342 (rev a1)
> > >         Subsystem: NVIDIA Corporation Device 16eb
> > >         Physical Slot: 0-5
> > >         Flags: bus master, fast devsel, latency 0, IRQ 263, NUMA node 0,  
> > IOMMU group 19  
> > >         Memory at 661002000000 (64-bit, prefetchable) [size=16M]
> > >         Memory at 662000000000 (64-bit, prefetchable) [size=128G]
> > >         Memory at 661000000000 (64-bit, prefetchable) [size=32M]
> > >
> > > I suppose this answers the BAR sizing question as well?  
> > 
> > Does this BAR2 size match the size we're reporting for the region?  Now
> > I'm confused why we need to intercept the BAR2 region info if there's
> > physically a real BAR behind it.  Thanks,
> >   
> 
> same confusion.
> 
> probably vfio-pci-core can include a helper for cfg space emulation
> on emulated BARs to be used by all variant drivers in that category?
> 
> btw intel vgpu also includes an emulation of BAR sizing. same for
> future SIOV devices. so there sounds like a general requirement but
> of course sharing it between vfio-pci and mdev/siov would be more
> difficult.

Yes, I see a need for this in the virtio-vfio-pci driver as well.  It
would simplify config emulation a lot if the variant driver could
manipulate the perm_bits.virt and .write bit arrays and simply update
vconfig for things like device-id and revision.  Thanks,

Alex