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

[David Matlack]

On Wed, Sep 16, 2026 at 11:58 AM Pratyush Yadav <pratyush@xxxxxxxxxx> wrote:
>
> 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.

Agree we will probably never add support for 32-bit to KEXEC_HANDOVER
and PCI_LIVEUPDATE. But I do prefer explicitly documenting the
dependency. It costs nothing and makes the 64-bit requirement crystal
clear, just in case someone tries to do something crazy :)