Alexander Graf kindly shared with the wolrd the link[1] to an informative article which among other things the article highlights the fact that QEMU's emulation of exception handling on x86[_64] is incorrect when the faulting instruction is RMW.
Here's small test case (__i386__ case is untested): #define _GNU_SOURCE #include <err.h> #include <signal.h> #include <stdarg.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <errno.h> #include <ucontext.h> #include <unistd.h> #define UNUSED __attribute ((unused)) /* http://fixunix.com/linux/7635-sigsegv-signal-handler.html */ static void handler (int signo, siginfo_t *info, void *context) { int iswrite; #ifdef __i386__ (void) context; iswrite = info.err & 2; #elif __x86_64__ struct ucontext *uc = context; (void) info; mcontext_t *mc = &uc->uc_mcontext; iswrite = mc->gregs[REG_ERR] & 2; #else #error shrug #endif if (iswrite) write (2, "write\n", 6); else write (2, "read\n", 5); _Exit (EXIT_FAILURE); } int main (int argc, char **argv) { int ret; enum {R,W,RMW} n = R; struct sigaction act; if (argc == 2) { if (!strcmp (argv[1], "read")) n = R; else if (!strcmp (argv[1], "write")) n = W; else if (!strcmp (argv[1], "rmw")) n = RMW; else errx (EXIT_FAILURE, "argument should be one of [read|write|rmw]"); } else { if (argc > 2) errx (EXIT_FAILURE, "need only one argument"); } act.sa_sigaction = handler; sigemptyset (&act.sa_mask); act.sa_flags = SA_SIGINFO; ret = sigaction (SIGSEGV, &act, NULL); if (ret) err (EXIT_FAILURE, "sigaction"); switch (n) { case W: asm ("mov %eax, 0"); break; case R: asm ("mov 0, %eax"); break; case RMW: asm ("incl 0"); break; } abort (); } Here's the output obtained on an x86_64 machine: rmw$ gcc rmw.c rmw$ ./a.out read read rmw$ ./a.out write write rmw$ ./a.out rmw write Since the last one is translated to more or less following TCG: movi rN, $0 ld32 rM, rN movi rL, $1 add rO, rM, rL st32 rO, rN and will incorrectly produce a read trap for both user and softmmu. [1] http://emulators.com/docs/nx33_qemu_0125.htm -- mailto:av1...@comtv.ru
