linux-kernel - Re: [PATCH v5 1/7] vfio/nvgrace-gpu: Use faults to map device memory

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Message-ID: <20251124171646.GU233636@ziepe.ca>
Date: Mon, 24 Nov 2025 13:16:46 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Shameer Kolothum <skolothumtho@...dia.com>
Cc: Ankit Agrawal <ankita@...dia.com>, Yishai Hadas <yishaih@...dia.com>,
	"kevin.tian@...el.com" <kevin.tian@...el.com>,
	"alex@...zbot.org" <alex@...zbot.org>,
	Aniket Agashe <aniketa@...dia.com>,
	Vikram Sethi <vsethi@...dia.com>, Matt Ochs <mochs@...dia.com>,
	"Yunxiang.Li@....com" <Yunxiang.Li@....com>,
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	Avihai Horon <avihaih@...dia.com>,
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	Alistair Popple <apopple@...dia.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Neo Jia <cjia@...dia.com>, Kirti Wankhede <kwankhede@...dia.com>,
	"Tarun Gupta (SW-GPU)" <targupta@...dia.com>,
	Zhi Wang <zhiw@...dia.com>, Dan Williams <danw@...dia.com>,
	Dheeraj Nigam <dnigam@...dia.com>,
	Krishnakant Jaju <kjaju@...dia.com>
Subject: Re: [PATCH v5 1/7] vfio/nvgrace-gpu: Use faults to map device memory

On Mon, Nov 24, 2025 at 05:09:30PM +0000, Shameer Kolothum wrote:
> > +static vm_fault_t nvgrace_gpu_vfio_pci_fault(struct vm_fault *vmf)
> > +{
> > +	struct vm_area_struct *vma = vmf->vma;
> > +	struct nvgrace_gpu_pci_core_device *nvdev = vma->vm_private_data;
> > +	int index = vma->vm_pgoff >> (VFIO_PCI_OFFSET_SHIFT -
> > PAGE_SHIFT);
> > +	vm_fault_t ret = VM_FAULT_SIGBUS;
> > +	struct mem_region *memregion;
> > +	unsigned long pgoff, pfn;
> > +
> > +	memregion = nvgrace_gpu_memregion(index, nvdev);
> > +	if (!memregion)
> > +		return ret;
> > +
> > +	pgoff = (vmf->address - vma->vm_start) >> PAGE_SHIFT;
> > +	pfn = PHYS_PFN(memregion->memphys) + pgoff;
> 
> The core fault code seems to calculate the BAR offset in vma_to_pfn()
> which is missing here.
> 
> pgoff = vma->vm_pgoff &
>                 ((1U << (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT)) - 1);

Yes, that should be included for this reason:

> Is the assumption here is user space will always map at BAR offset 0?

It should not be assumed.

Jason