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    SubjectRe: [patch] x86: convert rdtscll() to use __native_read_tsc

    * Ken Chen <kenchen@google.com> wrote:

    > Is there any reason why x86 rdtscll have to use the out of line function
    > instead of inline __native_read_tsc()? native_read_tsc and
    > __native_read_tsc is essentially the same functions.

    Your patch is correct.

    The reason for the __native_read_tsc() / native_read_tsc() distinction is
    and obscure problem with paravirt function pointers. Such constructs:

    ./xen/enlighten.c: .read_tsc = native_read_tsc,

    do not always work fine with all versions of gcc, if native_read_tsc() is
    a simple static inline (as it should be) - the build would fail with
    certain gcc flags. (and i remember runtime problems too) The C semantics
    of taking the address of an inline function seem pretty clear: the inlined
    function should be instantiated in that .o and a pointer should be
    generated out of that local instantiation.

    Perhaps the real fix is to do this rename as well:

    native_read_tsc => native_read_tsc_paravirt
    __native_read_tsc => native_read_tsc

    as this makes the native_read_tsc_paravirt() a pure technical variant, to
    be used in paravirt_ops function pointer assignments. People would thus
    just use the obvious native_read_tsc() inline function most of the time
    and could forget about native_read_tsc_paravirt().

    Jeremy?

    Ingo


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