Messages in this thread | | | Date | Mon, 09 Jun 2008 22:15:58 +0200 | From | Eric Dumazet <> | Subject | Re: [patch 00/41] cpu alloc / cpu ops v3: Optimize per cpu access |
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Andi Kleen a écrit : > Christoph Lameter <clameter@sgi.com> writes: > >> The problem is that offsets relative to %gs or %fs are limited by the >> small memory model that is chosen. > > Actually they are not. If you really want you can do > movabs $64bit,%reg ; op ...,%gs:(%reg) > It's just not very efficient compared to small (or rather kernel) model > and also older binutils didn't support large model. >
I am not sure Christoph was refering to actual instructions.
I was suggesting using for static percpu (vmlinux or modules) :
vmlinux : (offset31 computed by linker at vmlinux link edit time) incl %gs:offset31
modules : (offset31 computed at module load time by module loader) incl %gs:offset31
(If we make sure all this stuff is allocated in first chunk)
And for dynamic percpu :
movq field(%rdi),%rax incl %gs:(%rax) /* full 64bits 'offsets' */
I understood (but might be wrong again) that %gs itself could not be used with an offset > 2GB, because the way %gs segment is setup. So in the 'dynamic percpu' case, %rax should not exceed 2^31
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