linux-kernel - Re: [PATCH v4 3/3] vfio/nvgrace-gpu: Check the HBM training and C2C link status

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Message-ID:
 <SA1PR12MB7199DB6748D147F434404629B0E72@SA1PR12MB7199.namprd12.prod.outlook.com>
Date: Mon, 20 Jan 2025 02:24:14 +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>, Zhi Wang <zhiw@...dia.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>, Matt
 Ochs <mochs@...dia.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3/3] vfio/nvgrace-gpu: Check the HBM training and C2C
 link status


>> +EXPORT_SYMBOL_GPL(vfio_pci_memory_lock_and_enable);
>>
>>  void vfio_pci_memory_unlock_and_restore(struct vfio_pci_core_device *vdev, u16 cmd)
>>  {
>>       pci_write_config_word(vdev->pdev, PCI_COMMAND, cmd);
>>       up_write(&vdev->memory_lock);
>>  }
>> +EXPORT_SYMBOL_GPL(vfio_pci_memory_unlock_and_restore);
>>
>>  static unsigned long vma_to_pfn(struct vm_area_struct *vma)
>>  {
>
> The access is happening before the device is exposed to the user, the
> above are for handling conditions while there may be races with user
> access, this is totally unnecessary.

Right. What I could do to reuse the code is to take out the part
related to locking/unlocking as new functions and export that.
The current vfio_pci_memory_lock_and_enable() would take the lock
and call the new function. Same for vfio_pci_memory_unlock_and_restore().
The nvgrace module could also call that new function. Does that sound
reasonable?

> Does this delay even need to happen in the probe function, or could it
> happen in the open_device callback?  That would still be before user
> access, but if we expect it to generally work, it would allow the
> training to happen in the background up until the user tries to open
> the device.  Thanks,
>
> Alex

The thought process is that since it is purely bare metal coming to proper
state while boot, the nvgrace module should probably wait for the startup
to complete during probe() instead of delaying until open() time.

- Ankit Agrawal