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

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Message-ID:
 <CH3PR12MB7548F47FCF28EDF9EE0FB2B6ABD0A@CH3PR12MB7548.namprd12.prod.outlook.com>
Date: Mon, 24 Nov 2025 17:09:30 +0000
From: Shameer Kolothum <skolothumtho@...dia.com>
To: Ankit Agrawal <ankita@...dia.com>, "jgg@...pe.ca" <jgg@...pe.ca>, 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>
CC: "Yunxiang.Li@....com" <Yunxiang.Li@....com>, "yi.l.liu@...el.com"
	<yi.l.liu@...el.com>, "zhangdongdong@...incomputing.com"
	<zhangdongdong@...incomputing.com>, Avihai Horon <avihaih@...dia.com>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>, "peterx@...hat.com"
	<peterx@...hat.com>, "pstanner@...hat.com" <pstanner@...hat.com>, 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



> -----Original Message-----
> From: Ankit Agrawal <ankita@...dia.com>
> Sent: 24 November 2025 11:59
> To: Ankit Agrawal <ankita@...dia.com>; jgg@...pe.ca; Yishai Hadas
> <yishaih@...dia.com>; Shameer Kolothum <skolothumtho@...dia.com>;
> kevin.tian@...el.com; alex@...zbot.org; Aniket Agashe
> <aniketa@...dia.com>; Vikram Sethi <vsethi@...dia.com>; Matt Ochs
> <mochs@...dia.com>
> Cc: Yunxiang.Li@....com; yi.l.liu@...el.com;
> zhangdongdong@...incomputing.com; Avihai Horon <avihaih@...dia.com>;
> bhelgaas@...gle.com; peterx@...hat.com; pstanner@...hat.com; Alistair
> Popple <apopple@...dia.com>; kvm@...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: [PATCH v5 1/7] vfio/nvgrace-gpu: Use faults to map device memory
> 
> From: Ankit Agrawal <ankita@...dia.com>
> 
> To make use of the huge pfnmap support and to support zap/remap
> sequence, fault/huge_fault ops based mapping mechanism needs to
> be implemented.
> 
> Currently nvgrace-gpu module relies on remap_pfn_range to do
> the mapping during VM bootup. Replace it to instead rely on fault
> and use vmf_insert_pfn to setup the mapping.
> 
> Signed-off-by: Ankit Agrawal <ankita@...dia.com>
> ---
>  drivers/vfio/pci/nvgrace-gpu/main.c | 50 +++++++++++++++++------------
>  1 file changed, 30 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/vfio/pci/nvgrace-gpu/main.c b/drivers/vfio/pci/nvgrace-
> gpu/main.c
> index e346392b72f6..f74f3d8e1ebe 100644
> --- a/drivers/vfio/pci/nvgrace-gpu/main.c
> +++ b/drivers/vfio/pci/nvgrace-gpu/main.c
> @@ -130,6 +130,32 @@ static void nvgrace_gpu_close_device(struct
> vfio_device *core_vdev)
>  	vfio_pci_core_close_device(core_vdev);
>  }
> 
> +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);

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

> +
> +	scoped_guard(rwsem_read, &nvdev->core_device.memory_lock)
> +		ret = vmf_insert_pfn(vmf->vma, vmf->address, pfn);
> +
> +	return ret;

Could do return vmf_insert_pfn(vmf->vma, vmf->address, pfn); if
you don't need it later.

Thanks,
Shameer