Date: Mon, 29 Oct 2001 11:50:58 -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.011029115058.jhb@FreeBSD.org> In-Reply-To: <20011029143153.B14748@locore.ca>
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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? -- 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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