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    SubjectRe: [PATCH] x86: create array based interface to change page attribute
    Andi Kleen wrote:
    > Dave Airlie <airlied@redhat.com> writes:
    >
    >>
    >> +#define CPA_FLUSHTLB 1
    >> +#define CPA_ARRAY 2
    >>
    >
    > I don't think CPA_ARRAY should be a separate case. Rather single
    > page flushing should be an array with only a single entry. pageattr
    > is already very complex, no need to make add more special cases.
    >
    >> +
    >> + /*
    >> + * Only flush present addresses:
    >> + */
    >> + if (pte && (pte_val(*pte) & _PAGE_PRESENT))
    >> + clflush_cache_range((void *) *addr, PAGE_SIZE);
    >>
    >
    > Also it is doubtful clflush really makes sense on a large array. Just
    > doing wbinvd might be faster then. Or perhaps better supporting Self-Snoop
    > should be revisited, that would at least eliminate it on most Intel
    > CPUs.
    >
    >
    I agree that wbinvd() seems to be faster on large arrays on the
    processors I've tested. But isn't there a severe latency problem with
    that instruction, that makes people really want to avoid it in all
    possible cases?

    Also I think we need to clarify the semantics of the c_p_a
    functionality. Right now both AGP and DRM relies on c_p_a doing an
    explicit cache flush. Otherwise the data won't appear on the device side
    of the aperture.
    If we use self-snoop, the AGP and DRM drivers can't rely on this flush
    being performed, and they have to do the flush themselves, and for
    non-self-snooping processors, the flush needs to be done twice?

    /Thomas

    > -Andi
    >





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