From owner-freebsd-threads@FreeBSD.ORG Thu Apr 10 23:54:09 2003 Return-Path: Delivered-To: freebsd-threads@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 9F53737B401; Thu, 10 Apr 2003 23:54:09 -0700 (PDT) Received: from sccrmhc03.attbi.com (sccrmhc03.attbi.com [204.127.202.63]) by mx1.FreeBSD.org (Postfix) with ESMTP id BEBF843F93; Thu, 10 Apr 2003 23:54:08 -0700 (PDT) (envelope-from julian@elischer.org) Received: from interjet.elischer.org (12-232-168-4.client.attbi.com[12.232.168.4]) by sccrmhc03.attbi.com (sccrmhc03) with ESMTP id <2003041106540700300fd40de>; Fri, 11 Apr 2003 06:54:07 +0000 Received: from localhost (localhost.elischer.org [127.0.0.1]) by InterJet.elischer.org (8.9.1a/8.9.1) with ESMTP id XAA94859; Thu, 10 Apr 2003 23:54:06 -0700 (PDT) Date: Thu, 10 Apr 2003 23:54:04 -0700 (PDT) From: Julian Elischer To: Daniel Eischen In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII cc: David Xu cc: freebsd-threads@freebsd.org Subject: Re: patch for %gs saving X-BeenThere: freebsd-threads@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: Threading on FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 11 Apr 2003 06:54:10 -0000 On Fri, 11 Apr 2003, Daniel Eischen wrote: > On Thu, 10 Apr 2003, Peter Wemm wrote: > > > "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. > > BTW, it's not really a big deal for the UTS to restore %gs > (or probably whatever it ends up being on other archs) before > continuing a thread. I put it to you that %gs should be a way of finding the current KSE mailbox (upcall mailbox) which will NOT CHANGE when a thread is changed in userland. one simply changes the curthread pointer in the KSE (upcall) mailbox, leaving %gs the same. %gs should never change for a single KSE as it goes in and out of the kernel, it must always have the same mailbox. > > -- > Dan Eischen > > _______________________________________________ > freebsd-threads@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-threads > To unsubscribe, send any mail to "freebsd-threads-unsubscribe@freebsd.org" >