From owner-freebsd-arch Mon Oct 29 11:59:22 2001 Delivered-To: freebsd-arch@freebsd.org Received: from mail11.speakeasy.net (mail11.speakeasy.net [216.254.0.211]) by hub.freebsd.org (Postfix) with ESMTP id 9C80937B436 for ; Mon, 29 Oct 2001 11:59:13 -0800 (PST) Received: (qmail 90067 invoked from network); 29 Oct 2001 19:59:12 -0000 Received: from unknown (HELO laptop.baldwin.cx) ([64.81.54.73]) (envelope-sender ) by mail11.speakeasy.net (qmail-ldap-1.03) with SMTP for ; 29 Oct 2001 19:59:12 -0000 Message-ID: X-Mailer: XFMail 1.4.0 on FreeBSD X-Priority: 3 (Normal) Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 8bit MIME-Version: 1.0 In-Reply-To: <20011029145458.C14748@locore.ca> Date: Mon, 29 Oct 2001 11:59:01 -0800 (PST) From: John Baldwin To: Jake Burkholder Subject: Re: syscall() ABI questions Cc: arch@FreeBSD.ORG Sender: owner-freebsd-arch@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG On 29-Oct-01 Jake Burkholder wrote: > Apparently, On Mon, Oct 29, 2001 at 11:50:58AM -0800, > John Baldwin said words to the effect of; > >> >> On 29-Oct-01 Jake Burkholder wrote: >> > Apparently, On Mon, Oct 29, 2001 at 02:16:37AM -0800, >> > John Baldwin said words to the effect of; >> > >> >> 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? >> >> >> >> 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. >> > >> > The thing to watch for is that the system call wrappers don't follow >> > the same conventions for call-safe registers. At least on i386 vfork >> > stashes the return address in %ecx and expects it not to be clobbered >> > by the kernel. I think all the warppers that do this use %ecx (not %edx) >> > and its the same on sparc64, %o0 and %o1 are assumed to be clobbered >> > but other otherwise non-call safe registers are assumed to be preserved. >> > So it should be ok to always clobber retval[1] by setting it to zero. >> > >> > I think you're right about fork and rfork being able to use the MIASM >> > code. rfork with RFMEM is special but it can';t be safely called from >> > C anyway. The vfork wrapper needs to stay on x86 at least because both >> > processes return to the same stack; if the retunr address is not saved in >> > a register the child may clobber the parent's when it "rets" and pops >> > the stack. >> >> Same kernel stack? The register is set in the trapframe which means it is >> saved on the kernel stack. Is that shared in the vfork case? > > Same user stack. The trapframe is copied to the child's kernel stack. Then the vfork case should be fine, b/c we fixup %eax in the child's kernel stack so that when it returns from the syscall, %eax already has the right value. -- John Baldwin -- http://www.FreeBSD.org/~jhb/ PGP Key: http://www.baldwin.cx/~john/pgpkey.asc "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message