[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH] x86/altcall: further refine clang workaround
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 Regards, Roger.
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