linux-kernel - Re: drivers/pci: (and/or KVM): Slow PCI initialization during VM boot with passthrough of large BAR Nvidia GPUs on DGX H100

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Message-ID: <CAHTA-ua5g2ygX_1T=YV7Nf1pRzO8TuqS==CCEpK51Gez9Q5woA@mail.gmail.com>
Date: Thu, 5 Dec 2024 18:09:08 -0600
From: Mitchell Augustin <mitchell.augustin@...onical.com>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: linux-pci@...r.kernel.org, kvm@...r.kernel.org, 
	Bjorn Helgaas <bhelgaas@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: drivers/pci: (and/or KVM): Slow PCI initialization during VM boot
 with passthrough of large BAR Nvidia GPUs on DGX H100

I submitted a patch that addresses this issue that I want to link to
in this thread:
https://lore.kernel.org/all/20241206000351.884656-1-mitchell.augustin@canonical.com/
- everything looks good with it on my end.

-Mitchell Augustin


On Tue, Dec 3, 2024 at 5:30 PM Alex Williamson
<alex.williamson@...hat.com> wrote:
>
> On Tue, 3 Dec 2024 17:09:07 -0600
> Mitchell Augustin <mitchell.augustin@...onical.com> wrote:
>
> > Thanks for the suggestions!
> >
> > > The calling convention of __pci_read_base() is already changing if we're having the caller disable decoding
> >
> > The way I implemented that in my initial patch draft[0] still allows
> > for __pci_read_base() to be called independently, as it was
> > originally, since (as far as I understand) the encode disable/enable
> > is just a mask - so I didn't need to remove the disable/enable inside
> > __pci_read_base(), and instead just added an extra one in
> > pci_read_bases(), turning the __pci_read_base() disable/enable into a
> > no-op when called from pci_read_bases(). In any case...
> >
> > > I think maybe another alternative that doesn't hold off the console would be to split the BAR sizing and resource processing into separate steps.
> >
> > This seems like a potentially better option, so I'll dig into that approach.
> >
> >
> > Providing some additional info you requested last week, just for more context:
> >
> > > Do you have similar logs from that [hotplug] operation
> >
> > Attached [1] is the guest boot output (boot was quick, since no GPUs
> > were attached at boot time)
>
> I think what's happening here is that decode is already disabled on the
> hot-added device (vs enabled by the VM firmware on cold-plug), so in
> practice it's similar to your nested disable solution.  Thanks,
>
> Alex
>


-- 
Mitchell Augustin
Software Engineer - Ubuntu Partner Engineering