On 05.06.2025 18:59, Teddy Astie wrote: > Le 05/06/2025 à 12:26, Jan Beulich a écrit : >> --- a/xen/arch/x86/alternative.c >> +++ b/xen/arch/x86/alternative.c >> @@ -346,6 +346,12 @@ static int init_or_livepatch _apply_alte >> /* 0xe8/0xe9 are relative branches; fix the offset. */ >> if ( a->repl_len >= 5 && (*buf & 0xfe) == 0xe8 ) >> *(int32_t *)(buf + 1) += repl - orig; >> + else if ( IS_ENABLED(CONFIG_RETURN_THUNK) && >> + a->repl_len > 5 && buf[a->repl_len - 5] == 0xe9 && >> + ((long)repl + a->repl_len + >> + *(int32_t *)(buf + a->repl_len - 4) == >> + (long)__x86_return_thunk) ) >> + *(int32_t *)(buf + a->repl_len - 4) += repl - orig; > > That looks a bit confusing, to me that probably some comment explaining > what instructions transform it's looking to make.
It's still the same comment ahead of the if() that applies here. This will all become easier with Andrew's decode-light, at which point we'll be able to spot CALL/JMP anywhere in a blob. >> --- /dev/null >> +++ b/xen/arch/x86/memset.S >> @@ -0,0 +1,30 @@ >> +#include <asm/asm_defns.h> >> + > > It would be nice to have a reminder of the calling convention (i.e what > register maps to what memset parameter) as it will definitely help > future readers. > > If I understand correctly here : > - rdi: destination (s) > - rsi: byte to write (c) > - rdx: number of bytes to write (n) I don't think the (default) ABI needs re-stating for every function in every assembly file. >> +.macro memset >> + and $7, %edx >> + shr $3, %rcx >> + movzbl %sil, %esi >> + mov $0x0101010101010101, %rax >> + imul %rsi, %rax >> + mov %rdi, %r8 >> + rep stosq >> + or %edx, %ecx >> + jz 0f >> + rep stosb >> +0: >> + mov %r8, %rax >> + RET >> +.endm >> + >> +.macro memset_erms >> + mov %esi, %eax >> + mov %rdi, %r8 >> + rep stosb >> + mov %r8, %rax >> + RET >> +.endm >> + > > Overall, I am a bit confused on the mixing of 32-bits (edx, esi, ...) > and 64-bits registers (rax, ...). But it looks ok to me. Since 64-bit forms require REX, 32-bit ones are used wherever that's sufficient for the purpose. Jan
