Re: [Xen-devel] [PATCH] x86/vmx: fix build with clang 3.8.0

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

On Thu, Feb 09, 2017 at 06:14:54AM -0700, Jan Beulich wrote:
> >>> On 09.02.17 at 14:05, <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 09/02/17 13:01, Jan Beulich wrote:
> >>>>> On 09.02.17 at 13:49, <andrew.cooper3@xxxxxxxxxx> wrote:
> >>> On 09/02/17 11:33, Roger Pau Monne wrote:
> >>>> --- a/xen/include/asm-x86/hvm/vmx/vmx.h
> >>>> +++ b/xen/include/asm-x86/hvm/vmx/vmx.h
> >>>> @@ -602,15 +602,16 @@ void vmx_pi_hooks_assign(struct domain *d);
> >>>>  void vmx_pi_hooks_deassign(struct domain *d);
> >>>>  
> >>>>  /* EPT violation qualifications definitions */
> >>>> -typedef union __transparent__ ept_qual {
> >>>> +typedef union ept_qual {
> >>> Please can we use
> >>>
> >>> typedef __transparent__ union ept_qual {
> >>>
> >>> which clang is happy with, and will help avoid problems such as the
> >>> cper_mce_record issue in c/s f8be76e2fe
> >> Would clang also be happy with it moved near the end of that
> >> line
> >>
> >> typedef union ept_qual __transparent__ {
> >>
> >> Having the attribute ahead of "union" is, I think, strictly speaking
> >> undefined behavior, as it then may as well apply to "typedef".
> > 
> > No.  The result is
> > 
> > /local/xen.git/xen/include/asm/hvm/vmx/vmx.h:605:40: error: expected
> > identifier or '('
> > typedef union ept_qual __transparent__ {
> >                                        ^
> > /local/xen.git/xen/include/asm/hvm/vmx/vmx.h:614:3: error: type
> > specifier missing, defaults to 'int' [-Werror,-Wimplicit-int]
> > } ept_qual_t;
> >   ^~~~~~~~~~
> > 2 errors generated.
> > 
> > In which case the original patch as proposed will probably do.  It turns
> > out the presence of ept_qual_t does cause a compiler error if
> > __transparent__ is missing from scope.
> 
> But then the question is what the attribute applies to in the original
> version - the union, or just the typedef? The placement would
> suggest the latter, so I'd again be afraid of undefined behavior. Can
> it be moved ahead on that line?

This is what the clang folks seem to test:

https://github.com/llvm-mirror/clang/blob/master/test/Sema/transparent-union.c

So I would keep it with the current semantics, to stay in line with what they
do.

Roger.

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