linux-kernel - Re: [PATCH v2 02/10] PCI/LUO: Create requested liveupdate device list

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Message-ID: <20250930164705.GR2695987@ziepe.ca>
Date: Tue, 30 Sep 2025 13:47:05 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Chris Li <chrisl@...nel.org>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>, Len Brown <lenb@...nel.org>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-acpi@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
	Pasha Tatashin <tatashin@...gle.com>,
	Jason Miu <jasonmiu@...gle.com>, Vipin Sharma <vipinsh@...gle.com>,
	Saeed Mahameed <saeedm@...dia.com>,
	Adithya Jayachandran <ajayachandra@...dia.com>,
	Parav Pandit <parav@...dia.com>, William Tu <witu@...dia.com>,
	Mike Rapoport <rppt@...nel.org>, Leon Romanovsky <leon@...nel.org>
Subject: Re: [PATCH v2 02/10] PCI/LUO: Create requested liveupdate device list

On Mon, Sep 29, 2025 at 07:13:51PM -0700, Chris Li wrote:
> On Mon, Sep 29, 2025 at 10:47 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
> >
> > On Tue, Sep 16, 2025 at 12:45:10AM -0700, Chris Li wrote:
> > >  static int pci_liveupdate_prepare(void *arg, u64 *data)
> > >  {
> > > +     LIST_HEAD(requested_devices);
> > > +
> > >       pr_info("prepare data[%llx]\n", *data);
> > > +
> > > +     pci_lock_rescan_remove();
> > > +     down_write(&pci_bus_sem);
> > > +
> > > +     build_liveupdate_devices(&requested_devices);
> > > +     cleanup_liveupdate_devices(&requested_devices);
> > > +
> > > +     up_write(&pci_bus_sem);
> > > +     pci_unlock_rescan_remove();
> > >       return 0;
> > >  }
> >
> > This doesn't seem conceptually right, PCI should not be preserving
> > everything. Only devices and their related hierarchy that are opted
> > into live update by iommufd should be preserved.
> 
> Can you elaborate? This is not preserving everything, for repserveding
> bus master, only the device and the parent PCI bridge are added to the
> requested_devies list. That is done in the
> build_liveupdate_devices(), the device is added to the listhead pass
> into the function. So it matches the "their related hierarchy" part.
> Can you explain what unnecessary device was preserved in this?

I expected an exported function to request a pci device be preserved
and to populate a tracking list linked to a luo session when that
function is called.

This flags and then search over all the buses seems, IDK, strange and
should probably be justified.

Jason