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

["Teddy Astie" <teddy.astie@xxxxxxxxxx>]

Le 24/02/2026 à 12:25, Jürgen Groß a écrit :
> On 24.02.26 12:14, Roger Pau Monné wrote:
>> 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?
>
> No, it isn't.
>
> CONFIG_PVH is the common base needed for Xen and KVM guests to be able to
> run in PVH mode.
>

To me, CONFIG_PVH is more about being able to boot using PVH Direct Boot
than something else.

>>
>>> 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?
>>
>> It seems like a step backwards to merge this into some bigger generic
>> option, we always try to fine-grain as much as possible.
>>
>> Maybe you could introduce XEN_HVM meta option, that selects both PVHVM
>> and PVH?
> No, please don't use "HVM" for that purpose.
>
> If anything I'd set the CONFIG_XEN_PVH default to that of CONFIG_XEN_PVHVM.
>

That could work, but that would transitively imply that CONFIG_XEN needs
CONFIG_PVH, which I guess we probably want to avoid.

As I said, it's not required to boot with "PVH direct boot" for a "PVH
guest personality" to work in Linux since [1].

We can eventually consider decoupling CONFIG_XEN_PVH and CONFIG_PVH, but
that would break setup that expects that CONFIG_XEN_PVH implies
CONFIG_PVH (arch/x86/configs/xen.config in particular).

[1] 418492ba "x86/virt/xen: Use guest_late_init to detect Xen PVH guest"

>
> Juergen

Teddy


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Teddy Astie | Vates XCP-ng Developer

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