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Date: Sat, 30 May 2009 10:11:00 -0000
From: Eli Zaretskii <eliz@gnu.org>
Subject: System call support in process record and replay
To: Hui Zhu <teawater@gmail.com>
Cc: gdb@sourceware.org
Reply-to: Eli Zaretskii <eliz@gnu.org>
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I have a question about general design of the system call support for
the record/replay target, for systems whose system calls are entered
through software interrupts.

The following excerpt from i386-tdep.c shows the currently-only
implementation, for Linux system calls entered via INT 80h:

    case 0xcd:
      {
	int ret;
	if (target_read_memory (ir.addr, &tmpu8, 1))
	  {
	    if (record_debug)
	      printf_unfiltered (_("Process record: error reading memory "
				   "at addr 0x%s len = 1.\n"),
				 paddr_nz (ir.addr));
	    return -1;
	  }
	ir.addr++;
	if (tmpu8 != 0x80
	    || gdbarch_tdep (gdbarch)->i386_intx80_record == NULL)
	  {
	    printf_unfiltered (_("Process record doesn't support "
				 "instruction int 0x%02x.\n"),
			       tmpu8);
	    ir.addr -= 2;
	    goto no_support;
	  }
	ret = gdbarch_tdep (gdbarch)->i386_intx80_record (ir.regcache);
	if (ret)
	  return ret;
      }
      break;

Now, suppose there is another x86 target whose system calls are
entered through 3 software interrupts: 0x10, 0x21, and 0x31.  Does
this mean that to support such a target, we will need to define 3
additional members of `struct gdbarch_tdep', one each for every one of
the above interrupt numbers, and then tweak the above code to call
each member whenever the corresponding interrupt number is seen in the
instruction stream?  And adding support for Windows syscalls means
that yet another member, for INT 2Eh, should be added?  That seems
rather inelegant and wasteful to me (since these members will go
unused on every x86 target that does not use those interrupts), but if
that's the design we want to follow, I'm okay with it.