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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.04.2021 13:04, Andrew Cooper wrote:
> 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.

In which way? COMPAT gets selected by both PV32 and HVM.

Jan