Re: [RFC PATCH] x86/xen: Consider Xen PVH support in CONFIG_XEN_PVHVM

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Feb 24, 2026 at 12:34:31PM +0000, Teddy Astie wrote:
> Le 24/02/2026 à 12:16, Roger Pau Monné a écrit :
> > On Tue, Feb 24, 2026 at 10:51:35AM +0000, Teddy Astie wrote:
> >> It's currently possible to build Linux with CONFIG_PVH|CONFIG_XEN_PVHVM
> >> and no CONFIG_XEN_PVH. That leads to inconsistent kernels that fails with
> >> "Missing xen PVH initialization" when booting using PVH boot method or
> >> display various errors and fail to initialize Xen PV drivers when booting
> >> with PVH-GRUB.
> >>
> >>      platform_pci_unplug: Xen Platform PCI: unrecognised magic value
> >>      ...
> >>      # modprobe xen-blkfront
> >>      modprobe: ERROR: could not insert 'xen_blkfront': No such device
> >>      # modprobe xen-netfront
> >>      modprobe: ERROR: could not insert 'xen_netfront': No such device
> >>
> >> When built without CONFIG_XEN_PVH, PVH-specific logic is disabled, hence 
> >> when
> >> booting with e.g PVH-OVMF, Linux assumes we are a HVM guest, even when we 
> >> aren't
> >> actually one (in the "with HVM emulated devices" sense).
> >>
> >> As it is actually possible to boot Xen PVH without CONFIG_PVH; and that 
> >> most
> >> Xen-related logic exist within CONFIG_XEN_PVHVM; consider PVH guests 
> >> support
> >> within CONFIG_XEN_PVHVM instead of CONFIG_XEN_PVH.
> > 
> > So the current CONFIG_PVH selection done by CONFIG_XEN_PVH is moot?
> > 
> >> Keep CONFIG_XEN_PVH as a shortcut to enable PVH boot, ACPI support and 
> >> PVHVM.
> >>
> >> Signed-off-by: Teddy Astie <teddy.astie@xxxxxxxxxx>
> >> ---
> >> Cc: Juergen Gross <jgross@xxxxxxxx>
> >> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> >> Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> >>
> >> A tentative patch, I'm not sure of the way of dealing with the KConfig 
> >> part,
> >> keeping CONFIG_XEN_PVH as a shortcut is interesting, but there may be other
> >> options.
> >>
> >> There are widespreadly used Linux distributions that have a similar 
> >> configuration
> >> to this one, thus exhibit this issue i.e fail to boot.
> > 
> > Do you know the underlying cause of not enabling CONFIG_XEN_PVH?  Is
> > the default set to n on the defconfig?  Or are distros specifically
> > disabling this option on purpose?
> > 
> 
> I'm observing in these distros that
> 
>  > # CONFIG_XEN_PVH is not set
>  > CONFIG_XEN_PVHVM_GUEST=y
>  > CONFIG_XEN_PVHVM=y
> 
> Which makes CONFIG_XEN_PVH defaults to n.

We should possibly send a patch to those distros Kconfig to enable
CONFIG_XEN_PVH?  I think it's a bug on their side that Xen PVH is not
enabled.  Us trying to workaround this in our Linux Kconfig options
seem wrong.

Thanks, Roger.