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

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Message-ID: <BY5PR12MB376386DD53954BC3233AF595B0DEA@BY5PR12MB3763.namprd12.prod.outlook.com>
Date:   Wed, 25 Oct 2023 12:43:24 +0000
From:   Ankit Agrawal <ankita@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        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>,
        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

While the physical BAR is present on the device, it is not being used on the host
system. The access to the device memory region on the host occur through the
C2C interconnect link (not the PCIe) and is present for access as a separate memory
region in the physical address space on the host. The variant driver queries this range
from the host ACPI DSD tables.

Now, this device memory region on the host is exposed as a device BAR in the VM.
So the device BAR in the VM is actually mapped to the device memory region in the
physical address space (and not to the physical BAR) on the host. The config space
accesses to the device however, are still going to the physical BAR on the host.

> 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,

Yes, it does match the size being reported through region info. But the region
info ioctl is still intercepted to provide additional cap to establish the sparse
mapping. Why we do sparse mapping? The actual device memory size is not
power-of-2 aligned (a requirement for a BAR). So we roundup to the next
power-of-2 value and report the size as such. Then we utilize sparse mapping
to show only the actual size of the device memory as mappable.