Messages in this thread | | | Date | Thu, 10 Jul 2008 12:42:25 -0500 | From | Christoph Lameter <> | Subject | Re: [RFC 00/15] x86_64: Optimize percpu accesses |
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Jeremy Fitzhardinge wrote:
> If %gs:0 points to start of your percpu area, then all the offsets off > %gs are going to be no larger than the amount of percpu memory you > have. The gs base itself can be any 64-bit address, so it doesn't > matter where it is within overall kernel memory. Using zero-based > percpu area means that you must set a non-zero %gs base before you can > access the percpu area.
Correct.
> If the layout of the percpu area is done by the linker by packing all > the percpu variables into one section, then any address computation > using a percpu variable symbol will generate an offset which is > appropriate to apply to a %gs: addressing mode.
Of course.
> The nice thing about the non-zero-based scheme i386 uses is that setting > gs-base to zero means that percpu variables accesses get directly to the > prototype percpu data area, which simplifies boot time setup (which is > doubly awkward on 32-bit because you need to generate a GDT entry rather > than just load an MSR as you do in 64-bit).
Great but it causes trouble in other ways as discussed. Its best to consistently access per cpu variables using the segment registers.
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