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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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?

Thanks, Roger.