Re: [PATCH] x86/altcall: further refine clang workaround

["Alejandro Vallejo" <alejandro.vallejo@xxxxxxxxx>]

On Fri Jul 26, 2024 at 4:18 PM BST, Roger Pau Monné wrote:
> On Fri, Jul 26, 2024 at 03:25:08PM +0100, Alejandro Vallejo wrote:
> > On Fri Jul 26, 2024 at 3:17 PM BST, Alejandro Vallejo wrote:
> > > On Fri Jul 26, 2024 at 9:05 AM BST, Jan Beulich wrote:
> > > > On 26.07.2024 09:52, Roger Pau Monné wrote:
> > > > > On Fri, Jul 26, 2024 at 09:36:15AM +0200, Jan Beulich wrote:
> > > > >> On 26.07.2024 09:31, Roger Pau Monné wrote:
> > > > >>> On Thu, Jul 25, 2024 at 05:00:22PM +0200, Jan Beulich wrote:
> > > > >>>> On 25.07.2024 16:54, Roger Pau Monné wrote:
> > > > >>>>> On Thu, Jul 25, 2024 at 03:18:29PM +0200, Jan Beulich wrote:
> > > > >>>>>> On 25.07.2024 12:56, Roger Pau Monne wrote:
> > > > >>>>>>> --- a/xen/arch/x86/include/asm/alternative.h
> > > > >>>>>>> +++ b/xen/arch/x86/include/asm/alternative.h
> > > > >>>>>>> @@ -184,11 +184,11 @@ extern void alternative_branches(void);
> > > > >>>>>>>   * https://github.com/llvm/llvm-project/issues/82598
> > > > >>>>>>>   */
> > > > >>>>>>>  #define ALT_CALL_ARG(arg, n)                                   
> > > > >>>>>>>          \
> > > > >>>>>>> -    register union {                                           
> > > > >>>>>>>          \
> > > > >>>>>>> -        typeof(arg) e[sizeof(long) / sizeof(arg)];             
> > > > >>>>>>>          \
> > > > >>>>>>> -        unsigned long r;                                       
> > > > >>>>>>>          \
> > > > >>>>>>> +    register struct {                                          
> > > > >>>>>>>          \
> > > > >>>>>>> +        typeof(arg) e;                                         
> > > > >>>>>>>          \
> > > > >>>>>>> +        char pad[sizeof(void *) - sizeof(arg)];                
> > > > >>>>>>>          \
> > > > >>>>>>
> > > > >>>>>> One thing that occurred to me only after our discussion, and I 
> > > > >>>>>> then forgot
> > > > >>>>>> to mention this before you would send a patch: What if 
> > > > >>>>>> sizeof(void *) ==
> > > > >>>>>> sizeof(arg)? Zero-sized arrays are explicitly something we're 
> > > > >>>>>> trying to
> > > > >>>>>> get rid of.
> > > > >>>>>
> > > > >>>>> I wondered about this, but I though it was only [] that we were 
> > > > >>>>> trying
> > > > >>>>> to get rid of, not [0].
> > > > >>>>
> > > > >>>> Sadly (here) it's actually the other way around, aiui.
> > > > >>>
> > > > >>> The only other option I have in mind is using an oversized array on
> > > > >>> the union, like:
> > > > >>>
> > > > >>> #define ALT_CALL_ARG(arg, n)                                        
> > > > >>>     \
> > > > >>>     union {                                                         
> > > > >>>     \
> > > > >>>         typeof(arg) e[(sizeof(long) + sizeof(arg) - 1) / 
> > > > >>> sizeof(arg)];  \
> > > > >>>         unsigned long r;                                            
> > > > >>>     \
> > > > >>>     } a ## n ## __  = {                                             
> > > > >>>     \
> > > > >>>         .e[0] = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); 
> > > > >>> (arg); })\
> > > > >>>     };                                                              
> > > > >>>     \
> > > > >>>     register unsigned long a ## n ## _ asm ( ALT_CALL_arg ## n ) =  
> > > > >>>     \
> > > > >>>         a ## n ## __.r
> > > > >>
> > > > >> Yet that's likely awful code-gen wise?
> > > > > 
> > > > > Seems OK: https://godbolt.org/z/nsdo5Gs8W
> > > >
> > > > In which case why not go this route. If the compiler is doing fine with
> > > > that, maybe the array dimension expression could be further simplified,
> > > > accepting yet more over-sizing? Like "sizeof(void *) / sizeof (arg) + 1"
> > > > or even simply "sizeof(void *)"? Suitably commented of course ...
> > > >
> > > > >> For the time being, can we perhaps
> > > > >> just tighten the BUILD_BUG_ON(), as iirc Alejandro had suggested?
> > > > > 
> > > > > My main concern with tightening the BUILD_BUG_ON() is that then I
> > > > > would also like to do so for the GCC one, so that build fails
> > > > > uniformly.
> > > >
> > > > If we were to take that route, then yes, probably should constrain both
> > > > (with a suitable comment on the gcc one).
> > > >
> > > > Jan
> > >
> > > Yet another way would be to have an intermediate `long` to cast onto. 
> > > Compilers
> > > will optimise away the copy. It ignores the different-type aliasing rules 
> > > in
> > > the C spec, so there's an assumption that we have -fno-strict-aliasing. 
> > > But I
> > > belive we do? Otherwise it should pretty much work on anything.
> > >
> > > ```
> > >   #define ALT_CALL_ARG(arg, n)                                            
> > >   \
> > >       unsigned long __tmp = 0;                                            
> > >   \
> > >       *(typeof(arg) *)&__tmp =                                            
> > >   \
> > >           ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); })        
> > >   \
> > >       register unsigned long a ## n ## _ asm ( ALT_CALL_arg ## n ) = 
> > > __tmp; \
> > > ```
> > >
> > > fwiw, clang18 emits identical code compared with the previous godbolt 
> > > link.
> > >
> > > Link: https://godbolt.org/z/facd1M9xa
> > >
> > > Cheers,
> > > Alejandro
> > 
> > Bah. s/b/__tmp/ in line15. Same output though, so the point still stands.
>
> Had to adjust it to:
>
> #define ALT_CALL_ARG(arg, n)                                              \
>     unsigned long a ## n ## __ = 0;                                       \
>     *(typeof(arg) *)&a ## n ## __ =                                       \
>         ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); });         \
>     register unsigned long a ## n ## _ asm ( ALT_CALL_arg ## n ) = a ## n ## 
> __
>
> So that tmp__ is not defined multiple times for repeated
> ALT_CALL_ARG() usages.
>
> Already tried something like this in the past, but it mixes code with
> declarations, and that's forbidden in the current C standard that Xen
> uses:
>
> ./arch/x86/include/asm/hvm/hvm.h:665:5: error: mixing declarations and code 
> is incompatible with standards before C99 
> [-Werror,-Wdeclaration-after-statement]
>
> The `*(typeof(arg) *)&__tmp = ...` line is considered code, and is
> followed by further declarations.  Even if we moved both declarations
> ahead of the assigns it would still complain when multiple
> ALT_CALL_ARG() instances are used in the same altcall block.
>
> Thanks, Roger.

That _was_ forbidden in C89, but it has been allowed since. We have a warning
enabled to cause it to fail even if we always use C99-compatible compilers. I
think we should change that.

Regardless, I think it can be worked around. This compiles (otherwise
untested):

#define ALT_CALL_ARG(arg, n)
    register unsigned long a ## n ## _ asm ( ALT_CALL_arg ## n ) = ({   \
        unsigned long tmp = 0;                                          \
        *(typeof(arg) *)&a ## n ## __ = (arg);                          \
        BUILD_BUG_ON(sizeof(arg) > sizeof(void *));                     \
        tmp;                                                            \
    })

That said, if the oversized temp union works, I'm fine with that too.

Cheers,
Alejandro