Date: Mon, 29 Oct 2001 11:59:01 -0800 (PST) From: John Baldwin <jhb@FreeBSD.org> To: Jake Burkholder <jake@locore.ca> Cc: arch@FreeBSD.ORG Subject: Re: syscall() ABI questions Message-ID: <XFMail.011029115901.jhb@FreeBSD.org> In-Reply-To: <20011029145458.C14748@locore.ca>
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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 <jhb@FreeBSD.org> -- 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
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