[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [xen staging] x86/altcall: further refine clang workaround
commit 561cba38ff551383a628dc93e64ab0691cfc92bf Author: Roger Pau Monné <roger.pau@xxxxxxxxxx> AuthorDate: Wed Jul 31 12:41:22 2024 +0200 Commit: Jan Beulich <jbeulich@xxxxxxxx> CommitDate: Wed Jul 31 12:41:22 2024 +0200 x86/altcall: further refine clang workaround The current code in ALT_CALL_ARG() won't successfully workaround the clang code-generation issue if the arg parameter has a size that's not a power of 2. While there are no such sized parameters at the moment, improve the workaround to also be effective when such sizes are used. Instead of using a union with a long use an unsigned long that's first initialized to 0 and afterwards set to the argument value. Reported-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx> Suggested-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx> --- xen/arch/x86/include/asm/alternative.h | 26 ++++++++++++-------------- 1 file changed, 12 insertions(+), 14 deletions(-) diff --git a/xen/arch/x86/include/asm/alternative.h b/xen/arch/x86/include/asm/alternative.h index e63b459276..a86eadfaec 100644 --- a/xen/arch/x86/include/asm/alternative.h +++ b/xen/arch/x86/include/asm/alternative.h @@ -169,27 +169,25 @@ extern void alternative_branches(void); #ifdef CONFIG_CC_IS_CLANG /* - * Use a union with an unsigned long in order to prevent clang from - * skipping a possible truncation of the value. By using the union any - * truncation is carried before the call instruction, in turn covering - * for ABI-non-compliance in that the necessary clipping / extension of - * the value is supposed to be carried out in the callee. + * Clang doesn't follow the psABI and doesn't truncate parameter values at the + * callee. This can lead to bad code being generated when using alternative + * calls. * - * Note this behavior is not mandated by the standard, and hence could - * stop being a viable workaround, or worse, could cause a different set - * of code-generation issues in future clang versions. + * Workaround it by using a temporary intermediate variable that's zeroed + * before being assigned the parameter value, as that forces clang to zero the + * register at the caller. * * This has been reported upstream: * https://github.com/llvm/llvm-project/issues/12579 * 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; \ - } a ## n ## _ asm ( ALT_CALL_arg ## n ) = { \ - .e[0] = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); })\ - } + register unsigned long a ## n ## _ asm ( ALT_CALL_arg ## n ) = ({ \ + unsigned long tmp = 0; \ + BUILD_BUG_ON(sizeof(arg) > sizeof(unsigned long)); \ + *(typeof(arg) *)&tmp = (arg); \ + tmp; \ + }) #else #define ALT_CALL_ARG(arg, n) \ register typeof(arg) a ## n ## _ asm ( ALT_CALL_arg ## n ) = \ -- generated by git-patchbot for /home/xen/git/xen.git#staging
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