Re: [PATCH] xen/Kconfig: livepatch-build-tools requires debug information

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 07/11/2024 11:57 am, Jan Beulich wrote:
> On 07.11.2024 12:30, Andrew Cooper wrote:
>> On 07/11/2024 9:48 am, Jan Beulich wrote:
>>> On 07.11.2024 10:40, Roger Pau Monné wrote:
>>>> On Thu, Nov 07, 2024 at 09:21:26AM +0000, Andrew Cooper wrote:
>>>>> On 07/11/2024 8:49 am, Roger Pau Monne wrote:
>>>>>> The tools infrastructure used to build livepatches for Xen
>>>>>> (livepatch-build-tools) consumes some DWARF debug information present in
>>>>>> xen-syms to generate a livepatch (see livepatch-build script usage of 
>>>>>> readelf
>>>>>> -wi).
>>>>>>
>>>>>> The current Kconfig defaults however will enable LIVEPATCH without 
>>>>>> DEBUG_INFO
>>>>>> on release builds, thus providing a default Kconfig selection that's not
>>>>>> suitable for livepatch-build-tools even when LIVEPATCH support is 
>>>>>> enabled,
>>>>>> because it's missing the DWARF debug section.
>>>>>>
>>>>>> Fix by forcing the selection of DEBUG_INFO from LIVEPATCH.
>>>>>>
>>>>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>>> Oops, yes.
>>>>>
>>>>> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>>>
>>>>> Fixes tag ?
>>>> Was borderline on adding one, but wasn't sure since it's strictly
>>>> livepatch-build-tools that requires the DWARF data, but custom made
>>>> livepatches (like the examples in tests) do not require such
>>>> information.
>> Ok.  I guess it doesn't matter too much.
>>
>>> At which point: Is "select" really appropriate then? Wouldn't it be more
>>> logical then to change DEBUG_INFO's default to take LIVEPATCH into account
>>> (still permitting people to turn debug info off if they know they won't
>>> need it)?
>> I am very disinterested in letting people who think they can do
>> livepatching without debug symbols shoot themselves in the foot like that.
>>
>> It's only debugging symbols.   If people *really* think they know
>> better, they can strip them themselves.
> Besides my abstract concern, let me also add a concrete practical one. I'm
> sure you've noticed that xen.efi is _much_ slower to link with debug info
> than without, or than xen-syms. That's a consequence of how GNU ld (really:
> libbfd) works internally. By not allowing DEBUG_INFO to stay off you're
> forcing me to either wait longer for every single one of my post-commit
> pre-push build tests, or to turn off LIVEPATCH there. The latter not really
> being a good idea.
>
> Nevertheless, as said in reply to Roger: Go ahead if you absolutely think
> that's the only sensible way.

I had noticed that link was taking a long time.  I hadn't realised it
was this specifically.

But to put this another way, you're arguing to intentionally avoid
fixing a sharp corner, because there's a perf issue in Binutils.

I will note that it was you who forced the generation of xen.efi on
everyone, even those who didn't want it, without any knob to turn it off.

~Andrew