Re: [PATCH 4/4] x86/xen: Drop CONFIG_XEN_PVHVM_SMP

["Jürgen Groß" <jgross@suse.com>]

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On 05.08.26 11:04, Jan Beulich wrote:
> On 05.08.2026 10:55, Juergen Gross wrote:
>> On 05.08.26 10:44, Jan Beulich wrote:
>>> On 05.08.2026 10:21, Juergen Gross wrote:
>>>> --- a/arch/x86/xen/Makefile
>>>> +++ b/arch/x86/xen/Makefile
>>>> @@ -37,8 +37,8 @@ obj-$(CONFIG_XEN_PVH)		+= enlighten_pvh.o
>>>>    obj-$(CONFIG_EVENT_TRACING)	+= trace.o
>>>>    
>>>>    obj-$(CONFIG_SMP)		+= smp.o
>>>> +obj-$(CONFIG_SMP)		+= smp_hvm.o
>>>>    obj-$(CONFIG_XEN_PV_SMP)  	+= smp_pv.o
>>>> -obj-$(CONFIG_XEN_PVHVM_SMP)  	+= smp_hvm.o
>>>
>>> Similar issue here - in a PV-only config smp_hvm.o doesn't want / shouldn't
>>> need building.
>>
>> Note that I didn't change any functionality.
>>
>> I agree that it seems a little bit strange, but in the end I believe
>> the current status is okay-ish. PV-only hasn't been something in upstream
>> Linux since Xen support was added, as PV was always meant to be an
>> alternative to bare metal support via paravirt patching. It might have
>> been possible to build a kernel not really functional on bare metal, but
>> this was more like the ability to build a x86 kernel not working on any
>> existing machine.
>>
>> IMO the Xen kernel config options should allow for adding Xen-specific
>> features, but minimum Xen support should always have basic HVM support,
>> which includes the Xen specific performance enhancements.
> 
> I fear I don't understand this. If I want a kernel just to run as PV Dom0,
> why would it need to carry anything HVM-ish? That is (or should be)
> entirely unrelated to being able to also run this same kernel on baremetal
> then.

The fact is that Xen PV-mode was always a feature not really liked especially
by x86 maintainers (this is the polite way to phrase it).

Changing something outside of xen-specific parts of the kernel in favor of PV is
nearly always a fight and I'm pretty sure I only get changes in by playing nice
(not changing more than absolutely necessary and cleaning up constantly). I will
certainly not try to push for a "PV-only" kernel while the hope of the x86
maintainers is more "PV will go away some time in future".

The capability to configure the kernel without HVM stuff but with Xen support
is the way it has been since many years now. Nobody has missed a PV-only kernel,
so I conclude there is no need for that. The closest we will get here is a PVH
only kernel, and this is possible by not enabling CONFIG_XEN_PVHVM_GUEST.


Juergen

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