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    SubjectRe: [GIT PATCH] x86,percpu: fix pageattr handling with remap allocator
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    On Mon, 2009-05-18 at 12:41 -0700, H. Peter Anvin wrote:
    > Suresh Siddha wrote:
    > >
    > > Can we don't use PERCPU_DYNAMIC_RESERVE in the first chunk and for
    > > dynamic per_cpu_ptr's we can use some other offset such as
    > > per_cpu_ptr_offset() or some such thing?
    > >
    > > Then we can separate the static and dynamic chunks. And use large page
    > > kernel-direct mappings for fast access for critical common things and
    > > use small page accesses for dynamic and not so common accesses.
    > >
    > > Just checking to see if we can reduce the complexity of setting up the
    > > percpu areas (different versions for embed, non-embed etc) and handling
    > > all these aliases with simple code, rather than making it complex.
    > >
    >
    > I'm confused what you're suggesting here. The whole point of the percpu
    > unification work is that we can use %gs:absolute type references to hit
    > a variable right away. Although in theory we could use both %fs and %gs
    > for pointers, setting up both would greatly increase the cost of
    > entering the kernel, especially on 64 bits.
    >
    > This means all percpu data has to have a constant (virtual) offset from
    > the beginning of the static percpu area.

    This %gs:absolute type accesses are for static percpu data.

    But what I was referring to is the dynamic percpu data(accessed through
    per_cpu_ptr()). Instead of combining some part of the dynamic percpu
    data into the static percpu data(first percpu chunk), we can use
    different chunks for dynamic percpu data and governed by a different
    per_cpu_dynamic_offset[NR_CPUS] array.

    Then we can use large-page kernel direct mappings for static percpu data
    (or %gs:offset) and small-page vmalloc mappings for dynamic percpu data.

    thanks,
    suresh



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