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
