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

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Message-ID: <BN9PR11MB52767422419163F89D0DEC458CE72@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Mon, 20 Jan 2025 07:04:53 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Alex Williamson <alex.williamson@...hat.com>, Ankit Agrawal
	<ankita@...dia.com>
CC: Jason Gunthorpe <jgg@...dia.com>, Yishai Hadas <yishaih@...dia.com>,
	"shameerali.kolothum.thodi@...wei.com"
	<shameerali.kolothum.thodi@...wei.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>, "Sethi,
 Vikram" <vsethi@...dia.com>, "Currid, Andy" <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

> From: Alex Williamson <alex.williamson@...hat.com>
> Sent: Monday, January 20, 2025 11:23 AM
> 
> On Sun, 19 Jan 2025 20:12:32 -0700
> Alex Williamson <alex.williamson@...hat.com> wrote:
> 
> > On Mon, 20 Jan 2025 02:24:14 +0000
> > Ankit Agrawal <ankita@...dia.com> wrote:
> >
> > > >> +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?
> >
> > No, this is standard PCI driver stuff, everything you need is already
> > there.  Probably pci_enable_device() and some variant of
> > pci_request_regions().
> >
> > > > 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.
> >
> > If the driver is statically loaded, that might mean you're willing to
> > stall boot for up to 30s.  In practice is this ever actually going to
> > fail?  Thanks,
> 
> On second thought, I guess a vfio-pci variant driver can't
> automatically bind to a device, whether statically built or not, so
> maybe this isn't a concern.  I'm not sure if there are other concerns
> with busy waiting for up to 30s at driver probe.  Thanks,
> 

Can this wait be left to userspace i.e. the variant driver just does
one-off check and fail the probe if the device is not ready? Nvidia
can describe the requirement that the administrator may need to
wait for 30s to retry driver probe if the 1st attempt fails...