Re: [PATCH v8 03/12] PCI: liveupdate: Track incoming preserved PCI devices

[Pratyush Yadav]

On Wed, Sep 16 2026, David Matlack wrote:

> On Wed, Sep 16, 2026 at 11:25 AM Pratyush Yadav <pratyush@xxxxxxxxxx> wrote:
>>
>> On Wed, Sep 16 2026, David Matlack wrote:
>>
>> > On Tue, Sep 15, 2026 at 8:31 PM Zhu Yanjun <yanjun.zhu@xxxxxxxxx> wrote:
>> >> 在 2026/7/28 15:09, David Matlack 写道:
>> >
>> >> > CONFIG_64BIT is now required to enable CONFIG_PCI_LIVEUPDATE so that the
>> >> > domain and bdf can be guaranteed to fit in an unsigned long and be used
>> >> > as the xarray key.
>> >
>> >> > diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
>> >> > index 3781e2b5f095..8af20f558086 100644
>> >> > --- a/drivers/pci/Kconfig
>> >> > +++ b/drivers/pci/Kconfig
>> >> > @@ -273,7 +273,7 @@ config VGA_ARB_MAX_GPUS
>> >> >
>> >> >   config PCI_LIVEUPDATE
>> >> >       bool "PCI Live Update Support"
>> >> > -     depends on PCI && LIVEUPDATE
>> >> > +     depends on PCI && LIVEUPDATE && 64BIT
>> >>
>> >> One question about adding 64BIT to the dependency:
>> >>
>> >> As I understand it, enabling CONFIG_64BIT essentially means that we are
>> >> building a 64-bit kernel, and a 32-bit architecture cannot normally
>> >> enable CONFIG_64BIT.
>> >>
>> >> If that is the case, would depends on 64BIT be necessary here? Or is PCI
>> >> Live Update already inherently restricted to 64-bit architectures by the
>> >> existing LIVEUPDATE/architecture configuration, so that this dependency
>> >> would be redundant?
>> >>
>> >> If this problem has already discussed, I am very sorry about this.
>> >
>> > The necessity is that the PCI core needs to store more than 32-bits in
>> > the unsigned long xarray key (see the snippet above). The dependency
>> > on CONFIG_64BIT ensures that unsigned long is big enough. We could
>> > probably remove the dependency but I would rather wait until someone
>> > with a 32-bit build has a real use-case for using PCI_LIVEUPDATE
>> > before putting any effort into it.
>>
>> KHO or live update themselves don't support 32-bit platforms and there
>> are no plans to do so either. So I don't think you even need to have a
>> dependency on 64BIT in PCI_LIVEUPDATE. Only 64 bit architectures define
>> ARCH_SUPPORTS_KEXEC_HANDOVER, so PCI_LIVEUPDATE and others indirectly
>> inherit the dependency.
>>
>> If you'd like to be extra safe, then probably you should add a
>> dependency to 64BIT in KEXEC_HANDOVER directly. Though I think that can
>> be a separate patch independent from this series.
>
> It should be in both places then. PCI_LIVEUPDATE explicitly requires
> 64BIT for its own use of unsigned long, so it should have an explicit
> dependency. If KEXEC_HANDOVER requires 64-bit for its own specific
> purposes, then it should have an explicit Kconfig dependency as well.
> That way if and when someone wants to add 32-bit support we know
> exactly which configs need to add support.

Well, I don't think we are ever adding 32-bit support. Most 32-bit
support in the kernel is being deprecated or removed, so I really don't
think it makes much sense to extend KHO/live update to 32-bit
architectures.

But anyway, if you'd like to explicitly mark PCI as needing 64-bit, that
sounds fine too in the very very unlikely event we do add 32-bit
support.

-- 
Regards,
Pratyush Yadav