From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rusty Russell Subject: Re: Re: [patch 13/21] Xen-paravirt: Add nosegneg capability to the vsyscall page notes Date: Wed, 14 Feb 2007 16:38:46 +1100 Message-ID: <1171431526.19842.108.camel@localhost.localdomain> References: <20070213221729.772002682@goop.org> <20070213221830.466651996@goop.org> <45D23F97.5040907@vmware.com> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <45D23F97.5040907@vmware.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Zachary Amsden Cc: Andrew Morton , Jeremy Fitzhardinge , xen-devel@lists.xensource.com, virtualization@lists.osdl.org, linux-kernel@vger.kernel.org, Chris Wright , Ian Pratt , Christian Limpach , Andi Kleen List-Id: virtualization@lists.linuxfoundation.org On Tue, 2007-02-13 at 14:45 -0800, Zachary Amsden wrote: > Jeremy Fitzhardinge wrote: > > Add the "nosegneg" fake capabilty to the vsyscall page notes. This is > > used by the runtime linker to select a glibc version which then > > disables negative-offset accesses to the thread-local segment via > > %gs. These accesses require emulation in Xen (because segments are > > truncated to protect the hypervisor address space) and avoiding them > > provides a measurable performance boost. > > > > I don't like this because now a kernel compiled with both CONFIG_XEN and > CONFIG_VMI has "nosegneg" turned on. We don't actually require this for > performance or correctness, so it would be nice to be able to > dynamically turn it off instead of having it forced. Ditto for lguest. Rusty.