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

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Message-ID: <BY5PR12MB3763356FC8CD2A7B307BD9AAB0D8A@BY5PR12MB3763.namprd12.prod.outlook.com>
Date:   Mon, 23 Oct 2023 12:48:22 +0000
From:   Ankit Agrawal <ankita@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     Jason Gunthorpe <jgg@...dia.com>,
        Yishai Hadas <yishaih@...dia.com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        "kevin.tian@...el.com" <kevin.tian@...el.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>,
        Andy Currid <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

> 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.

> We also need to decide how strictly variant drivers need to emulate
> vfio_pci_config_rw with respect to BAR sizing, where the core code
> provides emulation of sizing and Yishai's virtio driver only emulates
> the IO port indicator bit.

Sorry, it isn't clear to me how would resizable BAR is applicable in this
variant driver as the BAR represents the device memory. Should we be
exposing such feature as unsupported for this variant driver?