Date: Thu, 10 Apr 2003 22:37:22 -0700 From: Peter Wemm <peter@wemm.org> To: "David Xu" <davidxu@freebsd.org> Cc: freebsd-threads@freebsd.org Subject: Re: patch for %gs saving Message-ID: <20030411053722.782152A7EA@canning.wemm.org> In-Reply-To: <008001c2ffe8$056584f0$f001a8c0@davidw2k>
next in thread | previous in thread | raw e-mail | index | archive | help
"David Xu" wrote: > Here is the patch for kernel to save %gs, > it works well on my machine. > http://people.freebsd.org/~davidxu/i386_gs.diff > Daniel, is this the reason in your libpthread > patch that doesn't use getcontext syscall ? To put some background on the issue, there is a reason why we did not do this. %gs is not used by the kernel, so it does not normally need to be saved and restored on every trap into the kernel. Setting a segment register is Really Slow - measured in hundreds of clock cycles. So, we normally only touch %gs when we context switch to a different process that may have a different %gs. Or when one of the context syscalls wants it changed. We cannot avoid touching %fs because we use it for kernel private data. But if it wasn't for that, we wouldn't be touching %fs for regular traps/syscalls/etc either. Bruce Evans understands this better than I do, I would suggest not making this change without talking about it with him first. Cheers, -Peter -- Peter Wemm - peter@wemm.org; peter@FreeBSD.org; peter@yahoo-inc.com "All of this is for nothing if we don't go to the stars" - JMS/B5
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20030411053722.782152A7EA>