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: <3aa3a726-147d-4573-ae50-eef94a910640@linux.dev>
Date: Mon, 1 Dec 2025 13:45:40 -0800
From: "Yanjun.Zhu" <yanjun.zhu@...ux.dev>
To: Pasha Tatashin <pasha.tatashin@...een.com>,
 David Matlack <dmatlack@...gle.com>
Cc: 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 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.

Yanjun.Zhu

> Please add the EXPORT_SYMBOL_GPL() to the public LUO API as needed, I
> was asked to stay conservative and not export until it is actually
> needed by modules.