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    SubjectRe: [patch 05/14] percpu: Use a Kconfig variable to configure arch specific percpu setup
    On Wed, 28 Nov 2007, Jeremy Fitzhardinge wrote:

    > > percpu references are quite frequent already (vm statistics) and will be
    > > more frequent after we have converted the per cpu arrays to per cpu
    > > allocations.
    > >
    > Well, I think the point is moot, because x86 will always use 32-bit
    > offsets. Each reference will only be 1 byte bigger than a normal
    > variable reference.

    Just because i386 is not able to use it does not mean that other arches
    are not. F.e. IA64 can embedd offsets in the actual instruction (but of
    course not 64bit).

    x86_64 can use a 32 bit offset instead of a 64 bit addres because it uses
    the small model. A load of a 64 bit address would require much more
    expensive instructions. A load of a 64 bit address is currently avoided
    through the use of the pda that contains the full 64 bit address in the
    data_offset field. Operations on per cpu data on x86_64 must therefore
    first load data_offset via gs and then add the per cpu address to this
    offset. Then the per cpu operation is performed on that address.

    In order to avoid this situation through one instruction we need a small
    32 bit offset relative to gs. Otherwise we cannot get away from the PDA
    and the use of data_offset.

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      Last update: 2007-11-29 03:09    [W:0.042 / U:8.708 seconds]
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