From d078d0aa86e9e3b937f673dc89306b3afd09d560 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Roger=20Pau=20Monn=C3=A9?= Date: Thu, 25 Jul 2024 16:21:17 +0200 Subject: [PATCH 56/56] x86/altcall: fix clang code-gen when using altcall in loop constructs MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Yet another clang code generation issue when using altcalls. The issue this time is with using loop constructs around alternative_{,v}call instances using parameter types smaller than the register size. Given the following example code: static void bar(bool b) { unsigned int i; for ( i = 0; i < 10; i++ ) { int ret_; register union { bool e; unsigned long r; } di asm("rdi") = { .e = b }; register unsigned long si asm("rsi"); register unsigned long dx asm("rdx"); register unsigned long cx asm("rcx"); register unsigned long r8 asm("r8"); register unsigned long r9 asm("r9"); register unsigned long r10 asm("r10"); register unsigned long r11 asm("r11"); asm volatile ( "call %c[addr]" : "+r" (di), "=r" (si), "=r" (dx), "=r" (cx), "=r" (r8), "=r" (r9), "=r" (r10), "=r" (r11), "=a" (ret_) : [addr] "i" (&(func)), "g" (func) : "memory" ); } } See: https://godbolt.org/z/qvxMGd84q Clang will generate machine code that only resets the low 8 bits of %rdi between loop calls, leaving the rest of the register possibly containing garbage from the use of %rdi inside the called function. Note also that clang doesn't truncate the input parameters at the callee, thus breaking the psABI. Fix this by turning the `e` element in the anonymous union into an array that consumes the same space as an unsigned long, as this forces clang to reset the whole %rdi register instead of just the low 8 bits. Fixes: 2ce562b2a413 ('x86/altcall: use a union as register type for function parameters on clang') Suggested-by: Jan Beulich Signed-off-by: Roger Pau Monné Reviewed-by: Jan Beulich master commit: d51b2f5ea1915fe058f730b0ec542cf84254fca0 master date: 2024-07-23 13:59:30 +0200 --- xen/arch/x86/include/asm/alternative.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/xen/arch/x86/include/asm/alternative.h b/xen/arch/x86/include/asm/alternative.h index 0d3697f1de..e63b459276 100644 --- a/xen/arch/x86/include/asm/alternative.h +++ b/xen/arch/x86/include/asm/alternative.h @@ -185,10 +185,10 @@ extern void alternative_branches(void); */ #define ALT_CALL_ARG(arg, n) \ register union { \ - typeof(arg) e; \ + typeof(arg) e[sizeof(long) / sizeof(arg)]; \ unsigned long r; \ } a ## n ## _ asm ( ALT_CALL_arg ## n ) = { \ - .e = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); }) \ + .e[0] = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); })\ } #else #define ALT_CALL_ARG(arg, n) \ -- 2.45.2