Date: Mon, 29 Oct 2001 14:42:14 +0000 (GMT) From: Doug Rabson <dfr@nlsystems.com> To: John Baldwin <jhb@FreeBSD.org> Cc: <arch@FreeBSD.org> Subject: Re: syscall() ABI questions Message-ID: <Pine.BSF.4.33.0110291437220.41783-100000@herring.nlsystems.com> In-Reply-To: <XFMail.011029021637.jhb@FreeBSD.org>
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On Mon, 29 Oct 2001, John Baldwin wrote: > I've got some questions about td->td_retval[1] and our syscall ABI. On some > archs (ia64, alpha) we preinitialie this value to 0. On other archs (i386, > sparc64, ppc) we set it to the value of the register it will be set to so that > effectively this register's value is preserved across the syscall. My question > is do our syscall ABI's actually assume that for syscalls with only one return > value that register isn't written to? NetBSD recently changed their i386 > syscall code to preinitialize to 0 rather than %edx. Anyone have the history > on this? For ia64 and alpha, it is safe to pre-initialise to zero. The register used in both cases is a scratch register. In ia64, there are actually four return value registers defined by the calling convention and we use the second one (r9) for td_retval[1] and the third one (r10) for the error flag. > > Speaking of i386, I have another question. For the fork, vfork, and rfork > syscalls, we have custom handlers that call the syscall normally and then > explicitly zero the return value if %edx is 1 (i.e., the child). However, in > vm_fork(), we already explicitly set the value in %eax to 0 for child > processes, so is this extra setting of that value in libc really needed? If > not, we can safely get rid of rfork.S, vfork.S, and fork.S in libc I think. Come to think of it, we probably can get rid of the fork wrappers. I haven't tried it though. -- Doug Rabson Mail: dfr@nlsystems.com Phone: +44 20 8348 6160 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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