linux-kernel - Re: [PATCH 00/21] vfio/pci: Base support to preserve a VFIO device file across Live Update

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Message-ID: <CALzav=fCjZnU5jqHTwFziAcssUys+XqWX1GkM7_PGufBnVyPmQ@mail.gmail.com>
Date: Mon, 1 Dec 2025 13:48:58 -0800
From: David Matlack <dmatlack@...gle.com>
To: "Yanjun.Zhu" <yanjun.zhu@...ux.dev>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>, Alex Williamson <alex@...zbot.org>, 
	Adithya Jayachandran <ajayachandra@...dia.com>, Alex Mastro <amastro@...com>, 
	Alistair Popple <apopple@...dia.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Chris Li <chrisl@...nel.org>, 
	David Rientjes <rientjes@...gle.com>, Jacob Pan <jacob.pan@...ux.microsoft.com>, 
	Jason Gunthorpe <jgg@...dia.com>, Jason Gunthorpe <jgg@...pe.ca>, Josh Hilke <jrhilke@...gle.com>, 
	Kevin Tian <kevin.tian@...el.com>, kvm@...r.kernel.org, 
	Leon Romanovsky <leonro@...dia.com>, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-pci@...r.kernel.org, 
	Lukas Wunner <lukas@...ner.de>, Mike Rapoport <rppt@...nel.org>, Parav Pandit <parav@...dia.com>, 
	Philipp Stanner <pstanner@...hat.com>, Pratyush Yadav <pratyush@...nel.org>, 
	Saeed Mahameed <saeedm@...dia.com>, Samiullah Khawaja <skhawaja@...gle.com>, Shuah Khan <shuah@...nel.org>, 
	Tomita Moeko <tomitamoeko@...il.com>, Vipin Sharma <vipinsh@...gle.com>, William Tu <witu@...dia.com>, 
	Yi Liu <yi.l.liu@...el.com>, Yunxiang Li <Yunxiang.Li@....com>
Subject: Re: [PATCH 00/21] vfio/pci: Base support to preserve a VFIO device
 file across Live Update

On Mon, Dec 1, 2025 at 1:46 PM Yanjun.Zhu <yanjun.zhu@...ux.dev> wrote:
>
>
> On 12/1/25 9:44 AM, Pasha Tatashin wrote:
> > On Mon, Dec 1, 2025 at 12:36 PM David Matlack <dmatlack@...gle.com> wrote:
> >> On 2025-12-01 05:32 PM, David Matlack wrote:
> >>> On 2025-12-01 09:16 AM, Zhu Yanjun wrote:
> >>>> 在 2025/12/1 9:10, David Matlack 写道:
> >>>>> On Mon, Dec 1, 2025 at 7:49 AM Zhu Yanjun <yanjun.zhu@...ux.dev> wrote:
> >>>>>> 在 2025/11/27 20:56, Zhu Yanjun 写道:
> >>>>>>> Hi, David
> >>>>>>>
> >>>>>>> ERROR: modpost: "liveupdate_register_file_handler" [drivers/vfio/pci/
> >>>>>>> vfio-pci-core.ko] undefined!
> >>>>>>>
> >>>>>>> ERROR: modpost: "vfio_pci_ops" [drivers/vfio/pci/vfio-pci-core.ko]
> >>>>>>> undefined!
> >>>>>>> ERROR: modpost: "liveupdate_enabled" [drivers/vfio/pci/vfio-pci-core.ko]
> >>>>>>> undefined!
> >>>>>>> ERROR: modpost: "liveupdate_unregister_file_handler" [drivers/vfio/pci/
> >>>>>>> vfio-pci-core.ko] undefined!
> >>>>>>> ERROR: modpost: "vfio_device_fops" [drivers/vfio/pci/vfio-pci-core.ko]
> >>>>>>> undefined!
> >>>>>>> ERROR: modpost: "vfio_pci_is_intel_display" [drivers/vfio/pci/vfio-pci-
> >>>>>>> core.ko] undefined!
> >>>>>>> ERROR: modpost: "vfio_pci_liveupdate_init" [drivers/vfio/pci/vfio-
> >>>>>>> pci.ko] undefined!
> >>>>>>> ERROR: modpost: "vfio_pci_liveupdate_cleanup" [drivers/vfio/pci/vfio-
> >>>>>>> pci.ko] undefined!
> >>>>>>> make[4]: *** [scripts/Makefile.modpost:147: Module.symvers] Error 1
> >>>>>>> make[3]: *** [Makefile:1960: modpost] Error 2
> >>>>>>>
> >>>>>>> After I git clone the source code from the link https://github.com/
> >>>>>>> dmatlack/linux/tree/liveupdate/vfio/cdev/v1,
> >>>>>>>
> >>>>>>> I found the above errors when I built the source code.
> >>>>>>>
> >>>>>>> Perhaps the above errors can be solved by EXPORT_SYMBOL.
> >>>>>>>
> >>>>>>> But I am not sure if a better solution can solve the above problems or not.
> >>>>>> I reviewed this patch series in detail. If I’m understanding it
> >>>>>> correctly, there appears to be a cyclic dependency issue. Specifically,
> >>>>>> some functions in kernel module A depend on kernel module B, while at
> >>>>>> the same time certain functions in module B depend on module A.
> >>>>>>
> >>>>>> I’m not entirely sure whether this constitutes a real problem or if it’s
> >>>>>> intentional design.
> >>>>> Thanks for your report. Can you share the .config file you used to
> >>>>> generate these errors?
> >>>>
> >>>> IIRC, I used FC 42 default config. Perhaps you can make tests with it. If
> >>>> this problem can not be reproduced, I will share my config with you.
> >>>>
> >>> What does "FC 42 default config" mean?
> >>>
> >>> Either way I was able to reproduce the errors you posted above by
> >>> changing CONFIG_VFIO_PCI{_CORE} from "y" to "m".
> >>>
> >>> To unblock building and testing this series you can change these configs
> >>> from "m" to "y", or the following patch (which fixed things for me):
> >> Oops, sorry, something went wrong when I posted that diff. Here's the
> >> correct diff:
> >>
> >> diff --git a/drivers/vfio/pci/Makefile b/drivers/vfio/pci/Makefile
> >> index 929df22c079b..c2cca16e99a8 100644
> >> --- a/drivers/vfio/pci/Makefile
> >> +++ b/drivers/vfio/pci/Makefile
> >> @@ -2,11 +2,11 @@
> >>
> >>   vfio-pci-core-y := vfio_pci_core.o vfio_pci_intrs.o vfio_pci_rdwr.o vfio_pci_config.o
> >>   vfio-pci-core-$(CONFIG_VFIO_PCI_ZDEV_KVM) += vfio_pci_zdev.o
> >> -vfio-pci-core-$(CONFIG_LIVEUPDATE) += vfio_pci_liveupdate.o
> >>   obj-$(CONFIG_VFIO_PCI_CORE) += vfio-pci-core.o
> >>
> >>   vfio-pci-y := vfio_pci.o
> >>   vfio-pci-$(CONFIG_VFIO_PCI_IGD) += vfio_pci_igd.o
> >> +vfio-pci-$(CONFIG_LIVEUPDATE) += vfio_pci_liveupdate.o
> >>   obj-$(CONFIG_VFIO_PCI) += vfio-pci.o
> >>
> >>   obj-$(CONFIG_MLX5_VFIO_PCI)           += mlx5/
> >> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> >> index c5b5eb509474..b9805861763a 100644
> >> --- a/drivers/vfio/vfio_main.c
> >> +++ b/drivers/vfio/vfio_main.c
> >> @@ -1386,6 +1386,7 @@ const struct file_operations vfio_device_fops = {
> >>          .show_fdinfo    = vfio_device_show_fdinfo,
> >>   #endif
> >>   };
> >> +EXPORT_SYMBOL_GPL(vfio_device_fops);
> >>
> >>   /**
> >>    * vfio_file_is_valid - True if the file is valid vfio file
> >> diff --git a/kernel/liveupdate/luo_core.c b/kernel/liveupdate/luo_core.c
> >> index 69298d82f404..c7a0c9c3b6a8 100644
> >> --- a/kernel/liveupdate/luo_core.c
> >> +++ b/kernel/liveupdate/luo_core.c
> >> @@ -256,6 +256,7 @@ bool liveupdate_enabled(void)
> >>   {
> >>          return luo_global.enabled;
> >>   }
> >> +EXPORT_SYMBOL_GPL(liveupdate_enabled);
> >>
> >>   /**
> >>    * DOC: LUO ioctl Interface
> >> diff --git a/kernel/liveupdate/luo_file.c b/kernel/liveupdate/luo_file.c
> >> index fca3806dae28..9baa88966f04 100644
> >> --- a/kernel/liveupdate/luo_file.c
> >> +++ b/kernel/liveupdate/luo_file.c
> >> @@ -868,6 +868,7 @@ int liveupdate_register_file_handler(struct liveupdate_file_handler *fh)
> >>          luo_session_resume();
> >>          return err;
> >>   }
> >> +EXPORT_SYMBOL_GPL(liveupdate_register_file_handler);
> >>
> >>   /**
> >>    * liveupdate_unregister_file_handler - Unregister a liveupdate file handler
> >> @@ -913,3 +914,4 @@ int liveupdate_unregister_file_handler(struct liveupdate_file_handler *fh)
> >>          liveupdate_test_register(fh);
> >>          return err;
> >>   }
> >> +EXPORT_SYMBOL_GPL(liveupdate_unregister_file_handler);
>
>
> Sure. Exactly. The above is the same with my solution. But after that
> EXPORT_SYMBOL_GPL, a cyclic dependency issue will occur.

Did see the change to drivers/vfio/pci/Makefile? That fixes the
circular dependency between vfio-pci and vfio-pci-core that I created
in this series.