From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rusty Russell Subject: Re: [PATCH 5/7] Use %gs for per-cpu sections in kernel Date: Mon, 25 Sep 2006 11:16:14 +1000 Message-ID: <1159146974.26986.30.camel@localhost.localdomain> References: <1158925861.26261.3.camel@localhost.localdomain> <1158925997.26261.6.camel@localhost.localdomain> <1158926106.26261.8.camel@localhost.localdomain> <1158926215.26261.11.camel@localhost.localdomain> <1158926308.26261.14.camel@localhost.localdomain> <1158926386.26261.17.camel@localhost.localdomain> <4514663E.5050707@goop.org> <1158985882.26261.60.camel@localhost.localdomain> <45172AC8.2070701@goop.org> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <45172AC8.2070701@goop.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.osdl.org Errors-To: virtualization-bounces@lists.osdl.org To: Jeremy Fitzhardinge Cc: Andi Kleen , lkml - Kernel Mailing List , virtualization List-Id: virtualization@lists.linuxfoundation.org On Sun, 2006-09-24 at 18:03 -0700, Jeremy Fitzhardinge wrote: > Rusty Russell wrote: > >> So are symbols referencing the .data.percpu section 0-based? Wouldn't = > >> you need to subtract __per_cpu_start from the symbols to get a 0-based = > >> segment offset? > >> = > > > > I don't think I understand the question. > > > > The .data.percpu section is the "template" per-cpu section, freed along > > with other initdata: after setup_percpu_areas() is called, it is not > > supposed to be used. Around that time, the gs segment is set up based > > at __per_cpu_offset[cpu], so "%gs:" accesses the local version. > > = > = > If you do > = > DEFINE_PER_CPU(int, foo); > = > then this ends up defining per_cpu__foo in .data.percpu. Since = > .data.percpu is part of the init data section, it starts at some address = > X (not 0), so the real offset into the actual per-cpu memory is actually = > (per_cpu__foo - __per_cpu_start). setup_per_cpu_areas() builds this = > delta into the __per_cpu_offset[], and so it means that the base of your = > %gs segment is at -__per_cpu_start from the actual start of the CPU's = > per-cpu memory, and the limit has to be correspondingly larger. Which = > is a bit ugly. Hi Jeremy! You're thinking of it in a convoluted way, by converting to offsets from the per-cpu section, then converting it back. How about this explanation: the local cpu's versions are offset from where the compiler thinks they are by __per_cpu_offset[cpu]. We set the segment base to __per_cpu_offset[cpu], so "%gs:per_cpu__foo" gets us straight to the local cpu version. __per_cpu_offset[cpu] is always positive (kernel image sits at bottom of kernel address space). > Especially since "__per_cpu_start" is actually very = > large, and so this scheme pretty much relies on being able to wrap = > around the segment limit, and will be very bad for Xen. __per_cpu_start is large, yes. But there's no reason to use it in address calculation. The second half of your statement is not correct. > An alternative is to put the "-__per_cpu_start" into the addressing mode = > when constructing the address of the per-cpu variable. I think you're thinking of TLS relocations? I don't use them... Rusty. -- = Help! Save Australia from the worst of the DMCA: http://linux.org.au/law