Re: [PATCH] x86/oprof: fix !HVM && !PV32 build

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/04/2021 11:58, Jan Beulich wrote:
> On 23.04.2021 12:51, Andrew Cooper wrote:
>> On 23/04/2021 10:50, Roger Pau Monné wrote:
>>> On Fri, Apr 16, 2021 at 04:20:59PM +0200, Jan Beulich wrote:
>>>> On 16.04.2021 15:41, Andrew Cooper wrote:
>>>>> On 16/04/2021 09:16, Jan Beulich wrote:
>>>>>> clang, at the very least, doesn't like unused inline functions, unless
>>>>>> their definitions live in a header.
>>>>>>
>>>>>> Fixes: d23d792478 ("x86: avoid building COMPAT code when !HVM && !PV32")
>>>>>> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>>> I agree this will fix the build.  However, looking at the code, I'm not
>>>>> sure the original CONFIG_COMPAT was correct.  In particular, ...
>>>>>
>>>>>> --- a/xen/arch/x86/oprofile/backtrace.c
>>>>>> +++ b/xen/arch/x86/oprofile/backtrace.c
>>>>>> @@ -43,6 +43,7 @@ dump_hypervisor_backtrace(struct vcpu *v
>>>>>>      return head->ebp;
>>>>>>  }
>>>>>>  
>>>>>> +#ifdef CONFIG_COMPAT
>>>>>>  static inline int is_32bit_vcpu(struct vcpu *vcpu)
>>>>>>  {
>>>>>>      if (is_hvm_vcpu(vcpu))
>>>>> ... this chunk of logic demonstrates that what oprofile is doing isn't
>>>>> related to the Xen ABI in the slightest.
>>>>>
>>>>> I think OProfile is misusing the guest handle infrastructure, and
>>>>> shouldn't be using it for this task.
>>>> I'm afraid I consider this something for another day. Both the
>>>> original #ifdef and the one getting added here are merely
>>>> measures to get things to build.
>>> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>
>>> Without entering on the debate whether CONFIG_COMPAT is the correct
>>> conditional to use it's not making the issue any worse, and it will
>>> allow to unblock the build. We can discuss about the CONFIG_COMPAT
>>> stuff later.
>> I disagree.  Fixing this less effort than the time wasted arguing about
>> fixing it.
>>
>> But if you are going to insist on not fixing it, and putting in a patch
>> like this, then at a minimum, it needs to include a TODO comment stating
>> that the use of CONFIG_COMPAT is bogus and needs fixing.
> I disagree: It is (for now) just you saying this is bogus. The (ab)use
> of the handle infrastructure was there before. You could have sent a
> fix long ago, therefore, if you were thinking this needs fixing.

I only know it needed fixing because you didn't build test your change
in CI.  Don't make it out to be my fault I didn't spot this 6 months ago.

> I can
> see that you have good intentions, but orthogonal issues shouldn't be
> used to block necessary adjustments (and this applies to other pending
> build fixes as well).

You genuinely regressed things for 32bit HVM guests, with the
CONFIG_COMPAT change.

The code may have been using inappropriate interfaces to perform its job
before, but its actually broken now.

~Andrew