Re: [PATCH 0/2] xen: Fix PV-only build

[Juergen Gross <jgross@suse.com>]

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On 06.08.26 15:24, Jason Andryuk wrote:
> On 2026-08-06 08:00, Jürgen Groß wrote:
>> On 06.08.26 13:31, Jan Beulich wrote:
>>> On 06.08.2026 12:55, Juergen Gross wrote:
>>>> On 06.08.26 03:52, Jason Andryuk wrote:
>>>>> Allow disabling XEN_PVHVM for a PV-only.  A stub in the event channel
>>>>> code needs to be fixed first.
>>>>>
>>>>> Jason Andryuk (2):
>>>>>     xen/events: Fix xen_set_upcall_vector stub
>>>>>     xen/Kconfig: select XEN_PVHVM
>>>>>
>>>>>    arch/x86/xen/Kconfig             | 8 +++++---
>>>>>    drivers/xen/events/events_base.c | 2 +-
>>>>>    2 files changed, 6 insertions(+), 4 deletions(-)
>>>>>
>>>>
>>>> I did a comparison of a kernel built with your patches disabling XEN_PVHVM
>>>> and my patches with XEN_PVHVM_GUEST disabled.
>>>>
>>>> The kernel built with my patches is 6 bytes smaller than the one with your
>>>> patches.
>>>
>>> Isn't this a sign of something else needing tweaking, somewhere?
>>
>> This is a sign that there are probably only very few really HVM specific paths
>> (in the sense of: explicitly not marked as irrelevant for PV) in the kernel.
>> Yes, I'm sure you can find some more, but I'm really not sure this is relevant
>> for more than a handful of users.
> 
> I see more reduction:
> 15029248 - arch/x86/boot/bzImage
> 15021056 - arch/x86/boot/bzImage.after
> 
> ~8k
> 
> 52542480 - vmlinux
> 52523184 - vmlinux.after
> 
> ~18k
> 
> $ ../linux/scripts/bloat-o-meter vmlinux vmlinux.after
> add/remove: 1/136 grow/shrink: 14/48 up/down: 15902/-22430 (-6528)

Did you disable CONFIG_XEN_PVHVM_GUEST in the before kernel, too?

> 
>>>> So I don't see any reason to take your patches, which conflict with mine,
>>>> especially as my patches have a negative diffstat on source level, too.
>>>
>>> Hmm, Jason's patches look to move things into a more adequate direction,
>>> though. In which case I think a negative diffstat becomes an irrelevant
>>> argument?
>>
>> Depends on what you are looking for.
>>
>> My take from this is that a PV-only kernel with Jason's patches is not really
>> adding any value, while my simplification is at least making things simpler
>> in terms of code volume and number of Xen related config options.
>>
>> Of course it would be possible to have a smaller PV-only kernel, but as I said
>> already, there has been no public demand for that in the last years and the
>> downsides IMHO far outweigh the potential gain.
>>
>> IMO the "adequate direction" regarding Xen specific kernel code is towards PVH
>> and not towards more PV specific tweaking. And I'm very sure the kernel
>> community outside of the Xen community is agreeing with me here.
> I don't need PV-only kernels, so I am fine with not pursuing this patch set.
> 
> Mainly I wanted to post this alternative since restoring PV-only is possible 
> (and I inadvertently broke it).  Converting CONFIG_XEN_PVHVM to CONFIG_XEN 
> looked wrong when it didn't apply to PV.
> 
> Pursuing your patches for the reasons you give also makes sense.

Thanks,


Juergen

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