From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Message-ID: <17517.2088.150094.338667@cargo.ozlabs.ibm.com> Date: Fri, 19 May 2006 09:50:00 +1000 From: Paul Mackerras To: "David S. Miller" Subject: Re: [RFC/PATCH] Make powerpc64 use __thread for per-cpu variables In-Reply-To: <20060510.171127.42619262.davem@davemloft.net> References: <20060510154702.GA28938@twiddle.net> <17506.29128.591758.502430@cargo.ozlabs.ibm.com> <17506.31456.68099.57515@cargo.ozlabs.ibm.com> <20060510.171127.42619262.davem@davemloft.net> Cc: linux-arch@vger.kernel.org, linuxppc-dev@ozlabs.org, linux-kernel@vger.kernel.org, amodra@bigpond.net, rth@twiddle.net List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , David S. Miller writes: > If you have to hide the operation so deeply like this, maybe you can > do something similar to sparc64, by explicitly doing the per-cpu fixed > register and offsets, and still get the single instruction relocs that > powerpc can do for up to 64K by doing something like: > > &per_cpu_blah - &per_cpu_base > > to calculate the offset. I don't know how to tell gcc that (&per_cpu_blah - &per_cpu_base) is a quantity that the linker can compute and that will fit into a 16-bit offset. If I use an inline asm, then I have to generate the address and let gcc dereference it, because __get_cpu_var is used both as an lvalue and an rvalue. That means two instructions where one would suffice. So there doesn't seem to be a way to get the optimal code, unless the gcc folks are willing to add a -fkernel or something for us. :) Paul.