Re: [PATCH] x86/shim: fix build when !PV32

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.05.2021 11:08, Roger Pau Monné wrote:
> On Fri, May 07, 2021 at 10:34:24AM +0200, Jan Beulich wrote:
>> On 07.05.2021 10:21, Roger Pau Monné wrote:
>>> On Fri, May 07, 2021 at 08:22:38AM +0200, Jan Beulich wrote:
>>>> In this case compat headers don't get generated (and aren't needed).
>>>> The changes made by 527922008bce ("x86: slim down hypercall handling
>>>> when !PV32") also weren't quite sufficient for this case.
>>>>
>>>> Try to limit #ifdef-ary by introducing two "fallback" #define-s.
>>>>
>>>> Fixes: d23d792478db ("x86: avoid building COMPAT code when !HVM && !PV32")
>>>> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>>
>>>> --- a/xen/arch/x86/pv/shim.c
>>>> +++ b/xen/arch/x86/pv/shim.c
>>>> @@ -34,8 +34,6 @@
>>>>  #include <public/arch-x86/cpuid.h>
>>>>  #include <public/hvm/params.h>
>>>>  
>>>> -#include <compat/grant_table.h>
>>>> -
>>>>  #undef virt_to_mfn
>>>>  #define virt_to_mfn(va) _mfn(__virt_to_mfn(va))
>>>>  
>>>> @@ -300,8 +298,10 @@ static void write_start_info(struct doma
>>>>                                            &si->console.domU.mfn) )
>>>>          BUG();
>>>>  
>>>> +#ifdef CONFIG_PV32
>>>>      if ( compat )
>>>>          xlat_start_info(si, XLAT_start_info_console_domU);
>>>> +#endif
>>>
>>> Would it help the compiler logic if the 'compat' local variable was
>>> made const?
>>
>> No, because XLAT_start_info_console_domU is undeclared when there are
>> no compat headers.
>>
>>> I'm wondering if there's a way we can force DCE from the compiler and
>>> avoid the ifdefs around the usage of compat.
>>
>> The issue isn't with DCE - I believe the compiler does okay in that
>> regard. The issue is with things simply lacking a declaration /
>> definition. That's no different from e.g. struct fields living
>> inside an #ifdef - all uses then need to as well, no matter whether
>> the compiler is capable of otherwise recognizing the code as dead.
> 
> Right, I see those are no longer declared anywhere. Since this is
> gating compat code, would it make more sense to use CONFIG_COMPAT
> rather than CONFIG_PV32 here?

I don't think so, no, from the abstract perspective that it's really
PV that the shim cares about, and hence other causes of COMPAT getting
selected shouldn't count.

Jan