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:<varname>" 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
